For the record, Liveaquaria's pictures do the species NO justice.
Generally as a loose rule of thumb, i like 15ish fish in a 55gal. Mbuna wise. In my 55gal i had 14 Msobo, a peacock male, a male Labeotropheus trewavasae, and a male Brichardi (had no where else to put him). And the tank did great. I found that if the peacock is large enough he can usually hold his own against the Mbuna....however peacocks are usually a lot more mild mannered then Mbuna so you never know.
More Zebras would be fine as well.