Just about any rainbow is a good fish for displays, but most are too big for your tank. Threadfins are a great choice as are any of the blue-eyes for a small aquarium. You could probably do a few M. praecox as another option in a small tank, depending on your fish load. Another option that I have always had luck with is emperor tetras; the males will do a fair amount of displaying if you get a good mix of m/f.
Keep in mind that the rainbows won't be at there best color and behaviour without lots of plants -- and they like a fair amount of current. Threadfins are also very succeptable to transport shock, so if you go that way, spend lots of time acclimating.
Keep in mind that the rainbows won't be at there best color and behaviour without lots of plants -- and they like a fair amount of current. Threadfins are also very succeptable to transport shock, so if you go that way, spend lots of time acclimating.