As much as I think mixing fish from different continents looks incorrect and goofy.
A friend of mine kept two pairs of adult festae in a 220 gal tank with about half dozen mbuna, they lived together happily for years. The festae pairs even spawned on opposite sides of the tank simultaneously a number of times.
I sometimes believe that the American cichlids don't perceive mbuna as cichlids, and thus not a threat.