I think for a short term home, a 55 can handle 4 bluegills. As the fish reaches sexual maturity, their growth is substantially slower. They will eventually reach a pretty tremendous size, several pounds is not unheard of. To minimize agression, you are gonna wanna keep all male or all female. If you want them to breed, you will need another tank to move the females to, and only keep one male. A trio would work, but once they get in the breeding mood, the male will kill anybody else in the tank.
Really, bluegills will require a larger tank in the long term. Even for one bluegill, a 55 is just not wide enough to be comfortable. I have personally handled a 16" bluegill, he was a monster.