i had 11 cardinals in a 15 and they didn't do much. they just hide behind the plants and sat there. same fish in a 55 and they are schooling and their behavior is awsome. I am just saying that fish behave differently in differently sized tanks. I personally would go with endler's or some guppies and some pigmy coryes, as someone else stated they are awsome little fish!!!!!!! Or you could do some small grouamis that would be interesting
you could get a few different species of small tetras in there. Neon, cardinals, golds, and greens look awesome together. I had a few of each in a 10 gallon for over a year, looked great. Just moved them into a 20, so now I can get some more.
You could just go with a very nicely colored pair of fish....say like the blue ram...very beautiful colors when happy and healthy...read up on them first though....
If you had a nice pair in there you could spruce up the tank with some live plants and interesting driftwood pieces. An otocinclus or two for cleaning as well. I think that would work out nicely. Even with only a few fish a tank can be very interesting and fun to observe if you do it right !