Dwarf gouramis (such as honey dwarfs) can be kept in a 2.5, no problem. You'd only want 1 in there, with maybe a few shrimp or a froggie or a snail. It doesn't need an air pump, but I don't think it would hurt anything. A small filter would probably be better, though. They can get oxygen out of the air, like bettas.
Dwarf gouramis are also passive, and they could go in with a molly (not in a 2.5, though, because mollies would outgrow it).