Welcome to the forum, Shane. It sounds like you are off to a great start.
Since you have a 6' tank, you have a bit of latitude in keeping the more aggressive mbuna in there. I would look to increase the size of the groups up to about 8-10 fish each. This will spread aggression and decrease your chances of cross-breeding.
For what you already have, I'd watch out for the potential crossing between the yellow labs, kenyi and red zebra. I have heard that the likelihood of yellow labs and red zebras interbreeding is fairly high in the aquarium. Kenyi and red zebra may be a concern as well (someone else chime in on that one).
If it were me, I'd cut out the middle man and ditch the red zebras. Other species that would work great in that tank would be rusties, Labeotropheus, Melanochromis (maingano or johani) and maybe a Cynotilapia.
Your mileage may vary with the P. crabro.... never tried keeping them in groups, but a 6' tank would certainly be the only way to go.