If I were to do a 20 long I would probably do charry shrimp (blue dream are my favorite) a couple amanos, 5 corys, 5 otos and as big a school of nano fish as I'm comfortable with. Pick something with colors that will pop against the plants. Ember tetras, chili rasporas, CPDs or plain old neons would all look great.
In a 20 tall I'd do the same except with guppies instead of a school of nano fish. They'll use the whole tank , and seeing babies in the tank is awesome. If you get to many babies you can take them to a local store or club, or let them control the population themselves.