Well, with a tank as big as 250 gallons, provided you have good water flow, heavy filtration, sufficient heating and enough cover in the form of lots of rock formations, you can pretty much get whatever mbuna you want. They certainly won't lack for space.
If you're interested in breeding them, the best thing to do is have a species tank - pick the exact species you want to breed, and stock only those fish. Mbuna have been known to interbreed, so if you wish to breed them it's best to keep only one kind.
If breeding's not your thing, then you need to decide how aggressive you'd like your tank stock to be. It's going to be somewhat aggressive no matter what you do - mbuna are all aggressive fish, even the relatively placid yellow labs. But then there are fish like demasoni, kenyi, auratus, and bumblebees, who are aggressive on a whole different level.
I don't mean this to seem like an interrogation, but what exactly are you looking for? What do you want the tank to end up like? Do you want fish of certain colours, or are their interactions and levels of aggression more of a priority?
Also, if you have any specific information on what type of filtration, powerheads, heating, and whatnot you'll be putting in the tank, that would help a lot with stocking advice.
