If I buy the Y-Wing Expansion and the Z-95 Expansion which are both Rebel expansions...and then buy the Most Wanted expansion (scum)...could I use the identities or whatever from the y-wing and z-95 expansion for the y-wing and z-95s in the most wanted pack as a way to pick up a couple of extra z-95s instead of buying the z-95 pack 3 times in a row?

Not exactly. The miniatures and the dials are interchangeable between the factions - as long as it's the right ship, then it doesn't matter if you use the Scum or Rebel versions. But the pilot cards and the cardboard base tokens aren't interchangeable. Rebels can't use Binyare Pirates, and Scum can't use Airen Cracken.

If you're playing casually, and your opponent is fine with remembering, "These are all Bandit Squadron Pilots, even though a couple of them say Binyare Pirate, they're all PS2," then it's no problem. But if you're playing in a competitive event, then all of your pilot cards and base tokens need to match your squad's faction.

But the pilot cards and the cardboard base tokens aren't interchangeable.

...although that will still leave a spare Z model.

Just buy four Rebel Z's and two copies of Most Wanted. Then you can fun 4 Bandits and 4 Talas! (That was my solution.)