Well, what are you hoping to keep in the tank? For an mbuna tank, the smallest recommended is a 55gal, but depending on what species you choose you can go smaller.
If you are thinking shell dwellers (Lake Tangyanika I believe) then you can do 20-30 tank I think (someone please confirm).
So basically, the smallest tank possible is dependent upon what you are hoping to keep.
yeah thats kinda the problem. also the reason i asked, should of been alittle clearer. i have an all male malawi 220g and a tang 75g at home. i wouldn't be comfortable keeping any of the fish i have there. i'm just not sure what else someone might have come up with. still open to all ideas. looks like a shell tank may be my only option....