One center piece, a group of school fish, and a group of bottom feeders.
Personally, I'd increase the number of corydoras catfish to 6. The more cory, the more active they will become. They are very cute when active. Not so cute when they are shy and hide all the time. Yes, I would choose the smaller species. Some of them can be 3.5" long which are a little big for a 20-gallon.
You may also increase the number of Neon Tetra to 10. Or get 10 Cardinal Tetra instead since they do not get the infamous Neon Tetra disease which has a high death rate.
One dwarf gourami is great as the center piece if you get it from a high quality fish place. Many dwarf gourami from cheap sources are also short lived.
A fully cycled 20-gallon fish tank should be able to handle 6+10+1 small tropical fish no problem, as long as you do not overfeed them.
For the top / middle level fish, I'd feed them New Life Spectrum pellets. You only need to feed them as much as they can finish all of it within 30 seconds. For the Corydoras, the best food is Hikia algae wafer, a single wafer per day is enough.
By the way, what aquarium filter are you using? The filter efficiency matters when it comes to how many fish you can have in the aquarium.
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20 gal tank - good mix of community fish ?
