Another option would be tropheus. 55 is about the minimum tank size you can keep them in- you basically pick a single species and get 15-20 of them, and they live in a big shoal-like colony. Unique behavior and interesting fish to own.
Good info GS, but personally I would never advise anyone to keep tropheus in a 55. Trophs require high quality water with high 02 levels, and due to their aggressive and territorial nature the longer the tank the better, so a 125 is really a much better fit for a beginner to that genus.