I feel the ac70 would be okay if you only plan on keeping 5 or less mbuna. The most I would stuff in a 55 would be 10, but you might want to add on another ac70.
I have 15 in my 55 gallon but it is heavily filtered with a strict water change schedule. I have heard of people keeping up to 20 individuals in that size tank.
On mbunas to keep the agrressiveness down the more the merrier(I have 70 fish in a 100 gallon) have adaquate filtration and weekly water changes and you should be ok .If you only keep for example 5 kenyi in a 55 you will soon have 1 big pretty yellow fish.
I have 14 mbunas in my 55g, running two AC 70's and do a weekly water change, haven't had any issues. It's all about good filtration, proper maintenence, & research before stocking.
LOL! having read this forum, I would say the accepted method is stock the tank then ask if the fish will get along okay!
Research before stocking is both prudent and highly recommended tho, so you are going about this well. It is good to buck a trend!