I've just noticed it (yeah, a bit late I suppose) - Scurrg in new X-wing will be scum only ship, right?

It seems like there won't be cross faction pilots. Did the devs say something about it?

This is correct and has been confirmed by devs. The firespray will also be scum only and no longer available to imperials.

Although I'd expect to still see some cross-faction pilots - it would be tricky not to have Fenn Rau in both Rebels and Scum for example. ?

No cross faction ships. Nothing said about cross-faction pilots, i assume they're still there.

What about y-wing?

Yeah, there'll be a few cross faction ships where it makes sense: HWK, Y-Wing, Z-95 etc. But no Rebel Nym.

Don't worry though, Sabine is still going to be able to fly everything.