I know these are technically rebel releases and coming with rebel only upgrades but does it say anywhere that the generic big ship is rebel only? Clearly the tantive would be rebels but a generic cr-90 could be imperials? I'm pretty sure in the EU both ships were used by both sides. Sorry if this is a dumb question.

Right now it looks like all the pilot cards for both ships have the RA logo on them, so they're rebel ships.

But there's no reason you couldn't play use a CR-90 with Tie Fighters if everyone in your group agreed to it. Technically you're not playing by the rules but it's not like you're giving unlimited torpedo's to rebel ships or something.