Hi there,
First off, what are you're water conditions (pH, GH & KH) as that will have a major influence on what you would be able to stock succesfully in such a small tank.
You'll struggle with Apisto's in a 15 gallon tank, ideally you want 20 gallons minimum for a pair (the male needs space to escape from the female when they breed). But in a 15 with plenty of hiding spaces you might just be lucky with a pair (and nothing else).
You could jump continents and look at some of the smaller shellies from Lake Tanganyika, a pair would probably work here as well (again, nothing else in the tank though).
Andrew