I agree with above. In a 20 gallon, the best cichlids would be shell dwellers.
Have you thought about bettas? Those are a fantastic fish for that size tank. I know they look all boring in the pet store, but give it good filtration, warm water (78ish), and he will be all over the place, he'll be your best friend! Mine came when he was called, flared at my finger, ate out of my hand, you name it. In a 20, you could also do a school of six otocinclus catfish, which in my opinion are the best algea eaters out there. They will keep the glass spotless.
If your set on cichlids, and all the more power to ya, shellies. If your just wanting to stock it, a betta and 6 oto's. You'll love it. PM me if that's the route you wanna go, I can give you lots of information and answer your questions if you'd like (or post here, there are a few betta-people lurking around).
-John
Edit: Let me just mention, the reason people think Betta is a boring species is this. In their native habitat, they have to "ride out" drought seasons, when they are only in a small amount of water (no, not puddles, not a horses hoofprint, pond size, as opposed to massive acres and acres of rice paddie), they slow down, breathe from the air, and have an uncanny ability to withstand massive amounts of ammonia/nitrite/nitrate, unlike any other fish. They don't do anything during this time period, they eat little, and conserve energy. That's what happens when they are in a bowl or a 2 gallon tank or something. Put him in a tank like yours, he'll think it's the rainy season and time to find a mate! They are very very active fish in the right conditions. Heat is important, as is filtration (if it's a cycled tank, he won't shut down to prevent damage, he'll run full steam!), trust me, they are great fish!