36 gallon is something like a 2.5 or 3 foot tank? I would say you could add more Africans with the zebs in that volume but you want to stick with other fish from Lake Malawi (there's many, many types of Malawi). Don't mix with Africans from Lake Tanganyika, most fish stores will generally have these in separate sections or at least tanks as they have differing needs with water, diet etc. Malawi's are the easier to keep and a bit less fussy then Tangs. They generally have a longer lifespan too.
Theory behind overstocking Africans is that the aggression is spread out amongst more fish so they all may have small disputes but they are less likely to have serious one on one targeted battles. That's not to say it always works out that way but that's the idea. Because you overstock them you do need fairly decent filtration though, they can handle a fair bit of water current (actually enjoy it) so that's not usually a problem.