A 55 will keep a bluegill alive but it might never reach full size. Bluegill are also pretty mean as far as sunfish go. I'll ask you to consider some of the other more colorful, more peaceful sunfish first. Almost all sunfish are more attractive than bluegill and many are better suited for home aquaria.
If you do decide on Bluegill, I'd put no more than one in a 55 since if you introduce two, one usually remains after some time.