Scholar, Somtimes I dont understand your logic hehehe.
Why would you suggest keeping platies in that tank? They need different foods, different water chemsitry, and will likely get thier butts kicked by the cichlids when they get larger.
I dont reccommend buying fish from EITHER petsmart or petco, they all too often carry hybrids and poor fish. Im not sure about in the states, but up here in Canada they are terrible. Buying from small LFS and private breeders are the best. Contact your local aquatic hobbyest club and try to find some african breeders in your area. That is by far your best bet, to both find what you want at reasonable prices and get healthy fish.
I think red zebras will be fine in the 55 gallon, however they are on the more aggressive side for the saulosi and yellow labs. If your worried about aggression possibly substitue the red zebras for socolofi, acei, or other less-aggressive fish. Having both yellow in the saulosi females and in the yellow labs might be a cause for concern, as the adult male yellow labs might chase the female saulosi, especially if you dont have enough females for the yellow labs. I've witnessed this when adding juvie saulosi in with adult yellow labs.
I agree with Scholar's Male to Female ratios. 4 of each species is good, and 1 male 3 females for each. 12 fish will be fine in the 55 gallon long term, and as mentioned you may need to weed out some extra males or over-aggressors.
But other than that, it sounds good! I would stick to mbuna since thats what your going for.... dont try to mix in haps with these guys, its best to do separate hap and mbuna tanks.

-Diana