I consider TIE's and X's to be the core ship for each faction with everything else being support options. For some reason I want to be able to have the option of running the "other" ships in pairs. I have no idea why I feel the need for pairs, I just do. So that's the way I'll be buying them. I'm not sure I'd like an all A-Wing, Y-Wing Reb Squad nor an All TIE Adv. list so it'll only be 2 each of them. But I'll probably be getting at least 4 TIE Interceptors before I'm done with the second wave.
Well, pairs is usually the base unit aircraft operate in, a lead and his wingman. Of course, Star Wars fighters seem to operate more like the early WW2 RAF, in groups of 3 (which were eventually dropped as being a bit rubbish and limiting, as they had been designed as bomber destroyer formations). Why they did escapes me, as those wingmen were rubbishly placed to provide cover against Darth Vader and his buddies (who also contributed exactly nothing to the whole endeavour. In fact, worse than nothing, as it was his panicking wingman that knocked him out of control).
Personally I would, if I had all the money in the world, want to field all the fighters in groups of 3, due to the film's thing of doing that. The exception would be the Tie Advanced, as that is a "squadron leader's" ship, and should be operating alongside regular tie fighters, rather than in units of its own. This again comes from their sole appearance in the films, and the fact that they would just look a bit silly in large numbers.