"Assorted africans" are usually mbunas, so I'll assume you are talking about mbunas. A 20 gallon tank is too small for them IMO. You usually need to have at least 12 or 15 to even out the aggression. Most of the common ones usually get to 5 or 6 inches long, so a standard 75 is really about the minimum to have a good group of them.
Edit: Lemon yellows dont get as big as most of the assorted africans, so you could have a nice group of 12 or 15 of those in a 50 or a 55.
Agree. The number of mbuna in a 20 you can comfortably house is zero. You can put shelldwellers from Lake Tanganika in a 20, but those are usually sold as shellies, not as assorted africans.
I was looking at either a 20 or 29 gallon tank. Any suggestions on what to populate it with. I already have one 75 gallon and was looking for a smaller tank for the dining room. Would Jewel Cichlid's work?