This bugged me endlessly in the Yoda event, but come on now - why lock tiers based on star level? I should be able to attempt the Emperor with my mismatched group of toons. If I get smoked, so be it, I'll know to level them up. Not being able to try though is incredibly frustrating.
My Rebels are:
7* Biggs ( thought the ship synergy would be something awesome, then they changed him)
4* STH, Lando, and Luke
I think I could at least take him out in the 5* star tier, can't enter though. Kinda stinks..