One red-tail black shark or rainbow shark would be fine... just don't get another fish with a similar body type, as the shark will be agressive towards it.
I'd also go with some of the larger (4") rainbows -many to choose from. And for the bottom I'd get a couple of loaches (yoyos or zebra) -stay away from clowns (too big) or skunks (too nasty). You could get cories instead of loaches, but not both as they'd be in competition in a tank that small. If you get loaches or cories, make sure they are not alone (at least 2 or 3 of same species).
Google for pictures.