Ok, mbuna cichlids is still not a species, but your getting closer. For a single species breeding tank you could use a 30 or 40 gallon with a 36"x18" footprint, the only difference between the 30 and 40 is the height.
Lots of different ways to go depending if you are aiming strictly at breeding for pure production or using a tank for pleasure with breeding if it happens. I'm more the accidental breeder type. I do pay attention to avoid mutts. With that, I find having tanks for grow out to be much more a problem. Thirty and up for many will work for the main tank but then lots of smaller tanks (10-20) for growing them to good size to sell. Right now, I have one 75 where the breeding occurs and four small growout and that is not enough for small tanks.