I would like to suggest rasboras. There are some really nice ones. Barbs too. Lots of activity and color.
If you want Cichlids, I know people will disagree, but a pair of convicts never ceases to be interesting, and they'll be able to shoehorn into a 10-20 gallon tank. They just won't reach full size of 6 inches. I love kribs, too, even though they aren't American.