Ok that's simple enough. The list you gave me that I said would work for your proposed 30 would definitely work in the 55, with even more. I doubt they'd hide much either, they tend to be quite confident in the right conditions (right parameters, low-ish lighting, lots of cover). Sand-bottomed tanks are great for mbunas, why don't you think it'd work?
Personally I'd go one or the other. Only mbunas, or only fish suitable to the requirements of the other fish you've mentioned.