I seem to mention them in any post about teeny tanks, but Heterandria formosa stay quite small; a couple of young ones would be fine in there for maybe six to eight months. Then even they get too big!
I agree-maybe two cherry shrimp and one of a small variety of snail (or juvenile of a larger snail) would be nice in there. If you have a tank that two young, tiny fish and a snail could graduate to then you could consider them, otherwise stay with two shrimp.