Not many options for a 20 gallon. If you wanted bigger fish than a bigger tank would be best. You might be able to get away with a dwarf gourami but I'm not sure how they would do with a ram.
You could do platys if you wanted to go livebearers, I wouldn't do mollies, they get nippy in that smaller space and swordtails would be out of the question.
Tetras or rasboras are really your best option.