FO Full Zeta Squad -- Who/When to replace?

Ditto. Fox replaces fotp. Also there are some good hybrid squads you can use to beat nightmare teams in arena. The lineup is kru, Fox, foo, rhan, thrawn. Stun vader with rhan, cast marching orders on fox and either use fox to dispel their tank if their tank has pretaunt or hit palp if not, kru stuns thrawn, thrawn fractures sion, then kill palp and Nihilus. Your kru with +30 speed from his leader ability must be faster than your opponents thrawn or your thrawn has to be faster than your opponents thrawn. If your kru is slower than your opponents thrawn but your thrawn is faster than your opponents, you can fracture thrawn and stun sion instead. Not as good though. If you face a team without a pretaunt it’s super easy. Fox does enough damage to palpatinne (and removes his tm) so that palp either dies or he’s forced to use his heal instead of stun.

Whatever you do though don’t leave the hybrid squad in on defense. It’s a purely offensive squad and will drop like a rock on defense.