The numbers I suggest in post #14 will not overstock the tank by any stretch. Provided you have floating plants, and do weekly partial water changes. [You will need a decent light, something we have not mentioned previously.]
First on the plants...these are mandatory for the surface fish but they also are frankly needed for the lower fish too. Forest fish (which is what we call almost all South American and many SE Asian fish common in aquariums) do not appreciate bright overhead light. Floating plants is an ideal way to shade the lower tank. Also, plants filter the water; fast-growing plants do this faster, and nothing is faster growing than floating plants. Floating plants that are growing are sometimes referred to as "ammonia sinks" because the amount of ammonia they can take up is phenomenal. And they are obviously using other nutrients from the water. So floating plants are a great benefit in any tank of such fish.
The other aspect of stocking involves the numbers. With shoaling fish, more is always better than too few. Because the fish individually will be less stressed the more there are, they actually have less of an impact biologically. The plants handle the ammonia well enough, but fish release other chemicals too that can only be removed with water changes, but the stress-inducing chemicals are fewer with more fish because they are less stressed.
The water changes are the third aspect; no less than 50-60% of the tank volume once each week. You really cannot change too much, but a once weekly change of 50-60% or even a bit more will do wonders for the stability of the biological system. And improve the fish even more, so there is still less impact.
You have a 20g long, and that is another benefit; within reason, you can have more fish in a 20g long than a standard 20g high. Primarily you have a larger surface area for the gas exchange. But the fish also have more space for the volume with a longer tank.
