Well, I have never been into the schooling fish very much, but once you get the water parameters settled down, you might look into glass catfish. THey are an amazing schooling fish. Who can't be interested in see-through fish. They like lots of current, so make sure you've got adequate filtration for them, and good clean water as well.
You can do a lot with a 20 gallon if you keep your fish small. My favorite tank is my 20-gallon long tank. Of course I planted mine like a jungle and then let it grow for a couple years. THe last time I did a major overhaul on it was last July when I moved cross country. Since then it's just grown thick. I should post a pic soon.
I just set up a regular 20 gallon tank. I think it's something like 24" long, 12" front to back, and 15" deep, whatever it is, I believe it's the standard common 20-gallon size. I got the setup for only $1 brand new in Petco minus a hood, which I didn't mind since I always custom make high-light hoods anyway.
SO, to get to my point here, if you wanted to do a few big fish, like I am doing with that tank, then you have some pretty cool fish you could go with, and bigger fish can have more character than smaller fish do. Currently I just have my synodontis euruptus in there with a couple of SAE for the inevitable algae build-up I always have in new high-light tanks. I've got mad light, and know I'll soon have mad algae, but it's a phase. The plan is for 2 discus, but my wife hates the thought of $80 fish. Go figure...