For your 20 gallon aquarium, what is the substrate going to be?
If it's something soft and smooth grained, such as sand or eco complete, then I would highly recommend corydoras catfish. Get 6 of the same species (typically the minimum number recommended), and you will have a happy little school on the bottom.
You could also instead do a school of 6 kuhli loaches. Make sure to provide lots of driftwood or rocks for spots for them to hide. Mine would pile up on top of each other and hide under the driftwood.
I would avoid danios and glofish, as they can be very nippy, and prefer cooler temperature water than a betta does.
Gouramis and guppies you want to avoid as well, along with tiger barbs.
I wouldn't recommend mollies, as they prefer harder water and a little marine salt in their aquariums for optimum health.
Possible (schooling) fish species would be small tetras (get a school of 6-8+)-neons, cardinals, glolight, lemon, pristella, black neons, etc, harlequin rasboras (6+), platies (at least 3, either all boys, all girls, or one boy to two girls ratio).
African dwarf frogs are a popular choice, but I've never had a set up with them and a betta work. Reason being, the frogs are such slow eaters, and betta's are little piggies. Even when using aquarium tongs to feed the frogs, my betta would swim on over and scare the frogs away, and then get the worms himself. Or, he wouldn't even scare the frogs away, just swim over and eat all the bloodworms before the frogs even saw them.
It resulted in hungry frogs, and a very fat betta. I eventually moved the betta to his own 10 gallon, and kept the frogs with male guppies.
If you do go the frog route, make sure you get at least two, as they are quite social. Habe some plants that reach near the surface of the tank for them to rest on as well. They are very cute, but I found them to be incredibly difficult to feed. They would literally be sitting in a pile of food, totally unaware. It was so frustrating.
Other people don't have issues with their ADF
's, so your experience might be different.
If you go with any of the smaller pleco species, keep in mind they have a heavy bioload, and that most need driftwood to eat for their diet, along with veggies.
A good set up in my opinion would be:
-1 male betta
-6 corydoras catfish or kuhli loaches
-8-10 neon/glolight tetras
I don't think you would be overstocked, and you would have both midwater and bottom water column species. A large school of tetras always makes for an impressive display as well.
Make sure to QT your new fish first, and add the betta last so he wont establish the entire tank as his territory.