OK. Remember, when dealing with Mbuna, you do so at your own risk. Based on my experience with with Pseudotropheus sp. "Long pelvic" and Melanochromis "maingano," I would go with three or four of the demasoni, and a half dozen or so Melanochromis.
Since the demasoni are blue, for the Melanochromis, I'd go with that old favorite, Melanochromis aureatus. Dark/black males with yellow/white striped females. Those should contrast very nicely with the blue demasoni.
Put lots and lots of rocks on a bare bottom tank, and build an artificial reef. Pile the rocks as high as you dare, without them toppling over. Then, put a half inch or so of sand on the bottom, just to cover it. (You don't want rocks on top of a lot of substrate, or your Mbuna may dig a hole under the rocks, which might cause them to fall and break the tank glass.
Also, before you put the rocks down, lay some egg crate on the bottom of the tank, to distribute the weight of the rocks, and as a barrier in case you're goofy enough to drop one. (It happens. Believe me, I know.)
The big high rock pile will make for an imposing tank decoration. It will be like a coral reef. The fish will hide, but will come out often enough to make things interesting--just like a coral reef tank.
Plus, the hiding places will probably (remember, there are no guarantees) keep your fish from killing each other.
