I think with the eventual (or existing) aggressiveness of the zebra's, your only choice could be a Jacobfreibergi of some kind. Then again this is still a slim chance. It all really depends on the overall temperament of your existing stock.
Wait, you already have 5 species in a 4 foot tank. If you are going to keep breeding groups, I wouldn't add any species, but instead, I would add females of the species you already have.
If you want an all male tank, remove all females and any extra males until you have one of each species. Then I might try some of the very aggressive peacocks, but as rsuarez1982 said, it may not work. You might have to remove the zebras and just keep the peacocks with the acei and labs.
Jakes for sure as the best Peacock with Mbuna but would be best to get these settled before adding the Mbuna. Not sure I would try these guys now (in a tank so small) as your Mbuna (and not the easiest going of the Mbuna) have settled.