
I sure hope we can help!
First. The stocking rule is 1" adult size fish per gallon of tank. - This means when you choose your baby fish, you have to reasearch how big they will get when they are full grown, and count up all the grown up inches........It makes for a sparce tank to begin with, but healthier for years to come!
Second, what kind of pleco do you have? If it a common, they don't fit in a 20 gallon tank. You should have a minimum of a 55 gallon tank. If it IS a common I would suggest taking it back to the store and trying to trade it for either a Bristlenose Pleco (bottom feeder, algae eater) or as part trade on 3 Dwarf panda corys (bottom feeder, eats all dropped food other fish miss) The corys have a super personality that your son would enjoy. The dwarf corys are a little more frail in my experience, but corys also need 3 or more and your tank isn't big enough with the other fish you have to accomodate the regular size fish.
The thing about Dwarf Gouramis is that you need 2 female and 1 male.....This cuts down on fighting. If you have this mix already I would keep them just the way they are and add more neons.
Neon tetras should be kept in schools of about 9-10, or more. Since your tank is only a 20 gallon, you may have to limit this, 7 would probably work.
This would take care of the top, middle, and bottom, as you want. I wouldn't try to get more gouramis or other kinds of fish until you get the neons up to a normal size school. They will be much happier.