A 55 gallon for africans would be a good choice, but they are aggressive. You need quite a few of them, like 8+, to distribute this aggression. This means that you need to carefully choose which africans you get - some get 12", others stay around 3-4". Go with smaller ones. You'll need LOTS of rocks for them to establish territories. This also means you'll need very good filtration on the tank to handle the bioload and frequent water changes are must.
IMO the zebras are not a good choice. They're one of the most aggressive and there are smaller options out there. 3-4 is not a good number. You'd probably need at least 6. Don't do this in a 30gal though. Instead of the zebras, I'd recommend something like yellow labs, demasoni, pseudotropheus saulosi.... But the fish you have currently would probably beat them to death. Hope this helps.