You could go with one Beta which is usually not aggressive with non-Betas, unless they look like a Beta, and I personally like mollies to go with it. A couple silver, a couple dalmatian, a couple black. To me that would be a nice 20 gal.
The silver molly is very bright, the black molly is very dark, and the dalmatian is a nice mix of both and can have either a black base or silver base with opposite spots that can be anywhere from lightly to heavily spotted. This to me would provide contrast (that is to me, colorful in itself) and a school of fish, that would make the vibrant colors and finnage of a nice beta stand out even more, being the solitary, larger fish. I personally like the blue and purple varieties that have fins similar to an angel fish.
The mollies love floating plants, live or fake. They will pick at stuff trying to help you keep things clean (I did say try

).
The beta would love you rescuing it from a sad existence in such small confines. A pointless piece of information about betas ... they have a labyrinth breathing system to where they can go to the surface and breathe air directly ... that is how that manage to survive in such horrid conditions. They are very happy with 2-3 gallons, but those little cups, small bowls, and vases are torture. If you have a snail problem, a beta will solve it! They love devouring snails.
Good luck and I hope you find
or someone suggests something that is right for you!!!