in ym experience with goldfish, you need 20g for the first fish, and 10 gal for every fish after that. but my goldfish outgrew that over time.
snails, dojo (wheather) loaches, would work good.
just to get an idea of how messy these guys are, a penguin 330 and 170 together couldnt keep the tank clean for a week (when they get older) i dunno how old yours are. mine were like 11cents and got 10-11 inches big