Most mbunas are tank raised. So just make sure your water isn't too different from the LFS, and you should be fine. Mbunas are a pretty hardy fish to begin with. I've only ever lost one because one got into a fight with the dominant one and lost pretty badly.
FW tanks don't need perfect levels in most cases. I've raised tetras in my hardwater and even had them spawn. With my mbunas, I never did any kind of adjusting with the water, and they've already spawned multiple times. Something must be right if they're doing that on a regular basis
. If you're worried about your soft water, just chuck a small bag of CC into a filter like was suggested before.
-Richer