Phoenix Rising I believe had fleets you could build as a unit and I did not like them (like most things from that mod, but my thoughts on PR are another matter). I found them generally to be pretty clunky and fairly pointless, it's a neat idea, but it doesn't really add anything to the game, on top of causing more issues than it's really worth.
My biggest problem with preset fleets is just that, they're preset. If you intend to use one in battle, you cannot adjust your fleet's composition on the fly to adapt to changing battle conditions, which is absolutely vital in long battles in EaW, and especially in ICW.
Plus, preset fleets play havoc with build limits, which the AI already ignores as a matter of course, but they allow the player to ignore them too. The reason for this is that, if a ship is destroyed in the fleet that has a build limit, the game thinks the ship is gone forever, so it subtracts from the build limit in order to allow you to build more. But if you have the fleet moving as a single unit on the galactic map, then the ships will always respawn the next time they go into battle unless the flagship is destroyed. This allows you to cheese and ignore build limits entirely by simply sacrificing the ship you want to build more of during a battle. Granted, I don't think this would be much of an issue in ICW, since only the largest and most powerful ships have build limits, but the problem is still there.
Basically, I think they're a neat idea, but EaW is simply too limited in what you can do with it to make them really worth having. Also, Gerfand, on the "fleet grouping" idea, I really like it, and I really wish that formation controls were something Petro had implemented in EaW. I would love to have something like that in the game, I can't tell you how many times I've needlessly lost ships during a battle simply because the game decided to spawn my fleet in an idiotic configuration with my light carriers out front and center for the AI to shoot at as I frantically order them to retreat to a more sensible position, etc. Unfortunately, though, I think that even if implementing something like this were possible, it would take far too much work.