PvP-Basic and PvP-Wolfpacks cover the concept of a Hydra fleet as it relates to frigates and destroyers. In Advanced, however, you'll moving on to larger ships, and so on to mixed fleet tactics. In Hydra our ships are fragile, relying too much on any individual ship is a significant risk. As a result, our tactics are all about contingency plans and covering up for weak links. In a heavier fleet we can afford to employ more specialised ships and more focused tactics, and the rewards for doing so can be significant.
However, so are the risks and so is the burden placed on each individual. If you're flying a Rifter in a hydra gang and you screw up your EWAR assignment then it doesn't tend to be a big deal because there are 4 other frigates there to take your place. But if you're the lone Falcon in a mixed fleet and you screw up your ECM assignment, then the entire fight can go differently. As a result, mixed fleets tend to be much more dependent on individual pilot skill than Hydra fleets. While Hydra fleets are mainly split up only between Skirmishers/Scouts and the main fleet, mixed fleets can have many different components, each of which is key to success. While a Basic class depends on having skilled skirmishers/cov-ops, a mixed fleet can depend on many more skilled individuals. To put it simply in order to function well in a mixed fleet you need to move beyond the simple model of a Basic (or Wolfpacks) class, and understand the inner workings of an EVE PvP fleet.
Size and Inertia
The biggest adjustment to make when moving from a Hydra gang to a mixed fleet is the increase in inertia. In a frigate fleet you can easily wait for an enemy fleet to land on grid before you warp away, or jump into a fleet then cold-warp to safety. With a mixed fleet however your larger ships will just be too big for maneuvers like this. As a result you need to look farther ahead, always preparing to do something before actually doing it. If you think an enemy fleet might be warping in to you then everyone in the fleet needs to be aligned and ready to warp well before they come in. As a general rule the larger ships you're in the slower your response time. This means that if you're 5 jumps away from your friends you might be fine with tackling a target because you know that your friends are in frigates and will be able to cover 5 jumps quickly. On the other hand if your friends are in a battleship fleet it will take likely close to 5 times as long to reach you. This means everyone in a mixed fleet needs to adjust their thinking. Skirmishers need to be ready to wait longer for backup. Cov-ops need to move out farther from the fleet to see threats sooner. The fleet commander needs to be planning ahead more, and give the fleet more time to execute maneuvers. If you treat a mixed fleet like a Hydra fleet then it's just a matter of time until things go badly.
A side point to take from this is that you should pretty much never ever chase a lighter fleet with a heavier fleet. The lighter fleet nearly always has the initiative when it comes to a fight. If the light fleet doesn't want to engage then the heavier fleet likely won't be able to catch them. Unless you have a good specific plan for how to catch a light enemy fleet then the most likely outcome is either them leading you into a trap, or them spreading you out and picking you off one by one. When in a heavier fleet go after other heavy fleets, or try getting light fleets come to you.
Any fleet without good recon is living on borrowed time. No matter what you're flying or how you're flying it, if you don't know what's happening around you it's just a matter of time until you get wiped out. A fleet's recon can come from a number of sources.
The first and most immediate is just use of readily available tools (namely local chat and the directional scanner) and requires no specialized ships or dedicated scouts. All players should constantly be keeping an eye on local and using the directional scanner to stay aware of what's happening around them. Whether you're the fleet commander, a skirmisher, or just another member of the fleet you should always know what's happening immediately around you, separate from any other recon you gain from other sources.
When you want to know more about what's happening than local and your scanner can tell you you need to get yourself a scout. Scouts can fall into 3 main categories, covert ops, skirmishers, and non-cloaking scouts. Covert ops are almost always covert ops frigates, but a Force Recon ship can work too if it comes to it. The main roll of a covert ops is to act as passive recon, usually getting into a system and staying there, reporting on a fleet's movements or watching a choke point. Covert ops are also perfect for quietly getting a warp-in point on a target that is not immediately available.
While a covert ops excels at being quiet and static, a skirmisher thrives on movement. If you ever have a skirmisher sitting still for longer than a couple minutes you are likely not using him to his full potential. The role of the skirmisher is to move a few jumps ahead of the fleet actively searching for targets and tackling them once he finds them. The best skirmishers tend to be long-range interceptors and interdictors due to the combination of their speed and their ability to tackle a target while remaining relatively safe. Skirmishers should never enter web range, and should avoid heavy neutralizer range if possible. The idea is to keep a point on a target while not allowing that target to hit you, so you need a microwarpdrive, a warp disruptor, and a speed setup. Damage output on a skirmisher is nice to have but a distant second priority to survivability. Skirmishers should be experienced with using the directional scanner to find targets and good at surviving in 0.0, even when jumping into hostile bubble camps. The more skirmishers an FC has the more he can allow them to roam and move freely. A skirmisher should not sit around waiting to be told what to do all the time, but should anticipate the FC's needs and accommodate them. Think of skirmishers as rabid dogs let loose to find an unsuspecting gazelle, tackle it, and gnaw on its leg until the rest of the fleet shows up.
Finally you have non-cloaking scouts. This group of ships is not as often seen since they are not as good of an option as a skirmisher or a covert ops. A non-cloaking scout would be any small fast ship that can evade enemies, but cannot effectively tackle on its own. When possible one should use a skirmisher instead, but if desperate a non-cloaking scout can work.
Key Components of a Fleet
What's the point in PvPing if you can't keep your target from running away? In a mixed fleet you want to have a good mix of tackling ships. Interceptors are always needed as skirmishers and primary tacklers (the first ship to get a point on a target) and should almost always be setup for range and speed in a fleet (so Ares, Raptor, Malediction, and Stiletto are preferred). In addition a couple of interdictors will give you a lot more flexibility not just in tackling but also in using bubbles as defensive measures. Heavy interdictors do have their uses, but in many ways are sub-par to a traditional interdictor in larger fleets. That being said it is still worth having one or two along if you expect to encounter super-capitals or camp a gate for an extended period. Finally Gallente and Minmatar Recons can be good to have around for extra range on scramblers and webifiers, which can make the difference between catching a fast ship and missing it. Once you have your specialized ships taken care of you'll probably want to round out your tackling by fitting tackle mods on normal ships. Having some larger ships setup with webs and scramblers is definitely a good idea because they can act as secondary tacklers and allow a lighter ship to warp out if it needs to, or to chase down other targets. T1 frigates can also function well here, utilizing an afterburner and small signature to orbit close and keep a target webbed and scrammed.
Once you have your target tackled and disabled with ewar then comes the fun part, doing damage! Just about any ship will deal damage, but when organizing a fleet you need to think about both how much damage you have, and the form that damage comes in. Pure DPS (damage per second) numbers are great and all, but typically have relatively little to do with winning a fight. Range and tracking are absolutely key to consider as well. For example, lets take a theoretical 5v5 battleship fight. One side puts out an average of 800 dps each, for 4000 dps total. The other side puts out 300 dps each for a total of 1500 dps. Which side will win? More often than not it would be team 2 who wins because they have longer range, allowing them to hit their enemy while not taking damage themselves. Do they do less damage? Sure, but they deal damage in a better way. Similarly you need to consider your tracking. A Hyperion may put out more theoretical dps than a Megathron, but often does less damage in practice because it lacks the tracking bonus that the Megathron has. All the dps in the world means nothing if you can't hit your target. Along similar lines, as you get into larger fleets you want to transition from using blasters or autocannons on battleships to using railguns and artillery with short ranged ammo. Yes, the tracking is worse, so you have to be careful, but the extra range allows you to put your damage on a target immediately, rather than having to fly into range. In larger fleets targets tend to die faster, so being able to put your damage on immediately allows you to do more total damage, even if you have less dps. The important thing here is to look at things from the perspective of the fleet as a whole. For a fleet you want consistency in range and diversity in tracking. Consistency in range is important, because you'll be warping as a fleet, so you might have 200 km max range, but if the rest of your gang is setup for <30km then you're going to be warping right on top of your targets and your extra range will be useless. You'd be better off fitting for closer range and performing that role better. Diversity in tracking is important so that you can adequately deal with smaller ships. Having a fleet full of tempests with 1400mm artillery might seem like a good idea until you run into a HAC/Recon gang. Always look at your fleet and think about how you'll deal with frigates (destroyers, other frigates are good counters), cruisers (HACs or BCs work well), and battleships (more damage is needed). You can get by fine with a number of good tracking ships (a HAC gang for example) as long as you have the damage to take down battleships in a reasonable time frame.
In Basic and Wolfpacks this category is basically limited to ewar and maintaining a good transversal. As you move along to larger ships and more advanced tactics though there are other options for dealing with incoming damage.
If they can't hit you they can't do damage to you. Being able to out-range your opponent can be extremely effective. Whether it's flying a vagabond and fighting a blaster ship, using your falloff to outrange his guns, or taking a sniper BS fleet against a short-range BS fleet, If you can control range you'll almost always win. The issue with relying on range though is that it tends to require fairly specialized setups, and lowers your fleet's damage output. This means that if you're setup for range, they're setup for close range and they get on top of you then you're in serious trouble. Sniping fleets make use of bookmarks, covert-ops drops, and TAMs in order to stay out at range, avoiding damage. If engaging at 150+ km pay special attention to any wrecks, either from one of your ships, or from killing an enemy tackler, because it can be used as a warp-in point for the enemy fleet.
Similar to using range, the key here is to avoid taking as much damage as possible. Outrunning your opponent's tracking can be very effective, but it's important to understand that signature radius plays a significant role as well. Many times you're actually better off turning off your MWD because the extra sig more than counteracts your extra speed, actually making you easier to hit. Outside of interceptors, interdictors, and choice other ships you very rarely want to orbit with a MWD on. Use your MWD to get into range, then turn it off, or use an AB if you want to orbit with extra speed.
When relying on speed and size you need to be very careful about how you fly. Getting into web range, or heavy neutralizer range can mean a very quick death, since speed ships tend to have weak tanks once they slow down. Similarly you're vulnerable to things like precision light missiles and warrior II drones, so speed tanking ships tend to require more experience than alternatives.
As a rule anymore you don't really have speed/size tanking fleets, outside of hydra fleets or interceptor gangs. In a mixed fleet you'll have your tacklers relying on speed and size while your heavier ships rely on other methods.
Instead of avoiding damage, why not just take it all then repair it? Repair tanking is actually very effective nowadays in EVE, but not in the way new PvPers might think. While missioning is very much about having a good, cap-stable personal repair tank, PvP is pretty much exactly the opposite. In PvP being cap-stable means very little, since you'll often be energy neutralize anyway. Instead it's better to fit a capacitor booster and rely on your boosters to keep your systems running. Additional person shield boosters and armor repairers are simply not very good in pvp. They just don't repair enough quickly enough to make a difference. Instead the dominant tactic in PvP is to rely on remote repair. By fitting 20 battleships each armor tanked and with a remote armor repair or two, you can have a much much larger tank on any given ship than if each of the battleships fitted its own armor repairer. Once you toss in a few logistics ships and you can have a quite amazing tank. The specifics of remote repair are beyond the scope of this class, but it's important to understand that it is a commonly used tactic, and often the most effective use of battleships in small to medium sized gangs. When fitting for remote repair you want to have a large amount of HP, as well as high resistances. As things are now remote repair only works well with large or capital repairers (meaning logistics, battleships, and carriers), and really only works well with armor tanking (more ships armor tank, plus the cpu usage on shield transfers is extreme). The most important thing though is that everyone in your fleet utilizes the same form of tanking, so that you can all repair each other. Also, it isn't worthwhile to try to repair light ships (frigates, destroyers) because they tend to die faster than you can repair them, outside of very small engagements.
As demonstrated in Basic and Wolfpacks, ewar is a key part to any pvp fleet. Whereas frigate and destroyer fleets work best with the Hydra principle, mixed fleets tend to have their ewar concentrated on specialized ewar ships. This is partially because the ewar bonuses on those ships give a significant effect, but also because specialized ewar ships are free to engage at range, rather than having to engage up close. You can still fit ewar to non-bonused ships in addition this this for extra effect, however this tends to be more focused rather than simple an even spread - for example damps on a kiting gang, or tracks on a close range gang. One key difference between mixed fleets and frigate/destroyer fleets is that target painters become very useful once you start adding ships that use medium and large sized guns. If you neglect to bring any target painters in a mixed fleet then you're severely cutting down on the amount of damage you can put out against smaller targets. Similarly, ship scanners are of less use in mixed fleets since you're not as concerned about smartbombs.