I just posted on another topic about compatibility.
Really there are almost no limits. I mean there are such things as snakes and mongoose in the fish-world, but it all comes down to the personality of the fish, and the tank itself. 55G gives your fish quite some breathing room if you don't overstock. Giving a fair amount of hiding places really help out a lot.
Some things to consider after you now which fish to buy is the landscape:
Fake plants/real plants
Live rock/plastic rocks
3-D background/picture background
Picking the fish(es) first really helps you decided on what to get
and there TONS of options!