Shelldwellers, yes, perfect for a 20 gallon (breeding pair of brevis, etc.)
Otherwise west africans (kribensis etc.).
20 gallon is awfully small for malawians as others have suggested... but if you really want to try it, try the only Malawian shell dweller (metriaclima livingstoni aka pseudotropheus lanisticola). they stay small (3-4"). and with enough care, a 20 gallon could support a breeding trio 1m/2f.