i think u said 'same fish-fish tank'. if you really mean it, i don't suggest gouramies. they get a little aggresive with each other when bigger(5 inches).
angelfish would be really nice... really...
tetras? depending on what kind. but a 55gal is a huge tank. i don't recommend fishes that are too small. it'd be hard to spot among the plants and/or driftwood sometimes. neons would be nice, a large school. but it'd be boring having small fish like that in a 'same fish-fish tank'. bigger fish would be nicer.
how bout average sized cichlids (u could get same species, it'd be nice. most ppl do traditional assorted cichlid tanks. dont have to do what they do. go for same species, try it out, it could be really nice.)
another option could be average sized carnivors. fish that grow to be 8-10 inches. u could house 3 or 4 in there.