I have a 20 gallon "tetra" tank that is exactly what you described Win. Tetras come in a ton of different colors shapes and sizes, it's not just the little neons and the black skirt tetras so commonly seen. Some fish are very active and can be nice to watch but will tear up any real plants you might put in there. So ask questions like that when you make your picks. You can choose up to about 13-15 fish with about 3-5 bottom feeders in a 20 gallon tank. I have 3 Panda Cory bottom feeders and two different types of pleco, a bristle nose and a clown pleco both of which are types that stay small. I have some Blue Columbian Tetras, some Glow Fish tetras, Diamond Tetra, Pristella tetra and some Neon Tiger Barbs which instead of black stripes they have white. You can mix and match tetra fish as long as they have similar body type for schooling purposes. Best bet for schooling fish is to do 2-3 min of whatever species you choose (the bottom feeders "don't count"). All of these fish tend to mix well with other "community" fish. Your first choice needs to be whether you're going natural or artificial for the rest of your tank, then you can go about making good choices of fish after that.
Also plan on setting up that tank for a good several days ( a week or two is good) before you bring in fish. The tank needs to have a "Bio bloom" before the fish come in. It will cloud up a bit for a few days before it clears back up. That's the tank biologically adjusting and becoming "natural" and a place where life can grow. Before that it's actually too sterile for fish and can shock fish into death.
Also HUGE WORD OF Caution...if you buy plants or even with fish, make sure you don't bring home any little snails from the petshop that hitchhike along for the ride. Your tank can become infested overnight and the only way to effectively get rid of em is to buy assasin snails. Long story...
Have fun!