so what do you have in there already? two yellow labs and two "electric blues"? you may be able to add a red zebra and another fish-but imo red zebras are quite aggressive-so make sure you have lots of rockwork.
yellow labs are quite docile and can be bullied by more aggressive fish. you could maybe get away with ONE red zebra-but keep an eye on it , and if its bullying your labs take it back. i love saulosis, rusties, blueberry OB zebras- hongi's--i could go on and on.
i wouldnt go over 6 or so fish-and i wouldnt mix too much either. if i had a 46 gallon tank-keep in mind that is small for mbuna- i would have yellow labs, rusties and saulosis mixed.. 3 species. it would look pretty great and they would all get along well.