Cichlids tend to be very hardy fish and can take just about any kind of water. In the "Great Salt Incident of 2002" (poor advice from the LFS to add a ton of salt to the tank) we lost a few cichlids, but most of them survived and have flourished since.
I've also heard that you can't mix african and south american, but i personally find that to be not true at all. We've got a mix of all sorts and those live in harmony. There are about 15 of them in a 55g tank, which they are VERY quickly outgrowing. They chase each other around, but its never gotten to the point where we had to take a fish out or anything of that sort. Once they get used to each other they can live quite peacefully. There is also a mono, an eel and some puffers in the tank and they get along quite well also.
I would say though that if your tank is only 20g go for good looking fish instead of alot of fish. You'll find they'll be much happier. They grow like its going out of style, especially if you feed them alot, so anticipate having to upgrade at some point.
Good luck!
-Dan