If you plan on keeping the fish for life I would recommend this...
Rockbass...only one. Think of them like coldwater oscars. Large, aggresive and messy.
Bluegill...probably only one for the same reason. Both these sunfish get very large.
Pumpkinseed...smallest of the three, maybe two or three of them in a 55.
There are many other smaller sunfish species as well such as longears, red brested, blue spotted and banded. For really small you could do a shoal of pygmy sunfish.