The above suggestion and advice sounds good. You could do a lot of different stuff with a 20gal. Do you know if it is a 20 High or 20 Long? Both are fine tanks for community fish, longs give you more possibilities.
You could stock a bit of an odd community. First, I suggest replacing the neon tetras with penguin tetras. They catch the eye and just look strange so naturally people look more closely. A dwarf gourami is a beautiful fish, but for this community a pair of pigmy gouramis works just fine. There are several types. I'd add a more overt splash of color with 4-6 male Endlers Livebearers, they will hang with the pigmy gouramis up top most the time. I would go with cory cats for the bottom, but add 2-4 khuli loaches in just for fun. This tank would be flush with flashy fish that all move in very different ways. Kind of like fish on a parking lot all moving different speeds and directions all at once.
Start with the tetras, add the gouramis and Endlers, then the bottom feeders. I'd go slowly, say at least a month.
