This is really a wide open question! You could stock a 75 with most africans, excepting the really large variety.
I've got several 'community' mbuna tanks that have a variety of mbuna and similar fish. These tanks have a certain visual appeal, but they require a bit more care in planning. (I try to avoid too much aggression and I try to avoid interspecies breeding. Feeding herbivores and omnivores in the same tank, and keeping both healthy, can also be a challenge.)
I've become more enamored of late with single species tanks. I have a 75 that has a large breeding colony of Ps. demasoni. The tank is quite striking, and the large number of these pugnacious fish seems to reduce the number of deaths to aggression. I like the fact that I can sell any offspring to the LFS with a clear conscience that none of the fish are mutts, too. Mostly, though, I like how they look. Next on my list: a cyrtocara moori tank.
The mix of fish really comes down to personal preference. The number of fish is generally dictated by whether you want to overstock or not. Overstocking is practiced to disperse aggression and is often successful in doing so, especially in tanks 75 g and larger, but it carries with it greatly increased demands for maintenance and filtration. I have a couple of overstocked tanks. It does help when keeping some of the more belligerent species (e.g., Ps. lombardoi), but you have to be ready for lots of filter cleaning and lots of water changes. Not something for a beginner to try, IMO.
HTH,
Jim