Never had any demonsoni, but just looked them up and now I want some!
Anyways, I have kept M. Auratus and P. Kennyi (3 of each) in the same tank with my current 6 yellow labs without too much trouble even after I was told millions of times that the auratus would tear everyone to shreds without fail. I kept all of these fish for about 5 months until the males started to change color and I found that ALL of the auratus were male (go figure, huh?).
Point being, after reading the demonsoni profile which says that you shouldnt be fooled into keeping more than one demonsoni in anything less that a 50 gallon tank, I am still quite sure that if you do things properly you can reduce the aggression of all the fish quite a bit. The best part about these cichlids is that it is not only acceptable to overstock a tank with them, it is often times recommended to do so!
I found that the one good trick for mixing differing levels of aggression in cichlids was to try to put as many caves, nooks and crannies into the tank as possible. This gives ample opportunity for each fish to get a little territory. It is also good to do your best to cut or narrow the 'sightlines' in a tank so that fish cannot see from one end to another. This keeps the territories more seperated from each other.
What I also found that REALLY helped was to use dither fish. In my cichlid tank (which now only holds yellow labs because I am trying to breed them) I have 8 rather large tiger barbs that school really well together and are just plain old too fast for the cichlids to catch. They also tend to stand up for themselves and nip back. At any rate, it works beautifully and it not only keeps the aggression spread out far and wide, it also keeps all my fish out in the open for the most part!
