Demasoni are very tough, very aggressive little fish.
Best way to keep these guys is in a heavily stocked species tank. Most of the aggression with them is conspecific, so whether in a species or community tank, the more of their own number there are, the less chance of fatal aggression. The general experience is that if you start off with only 3 or 4 or 5 of these guys you'll see them picked off one at a time until there's only one. In a 55 gallon I'd recommend about 15-20 in a species tank.
Because aggression is usually limited to within their group, a single specimen can usually be added to an mbuna community fairly safely, as long as there are no other mbuna too close in appearance.