I kept Demasoni for a long time and really enjoyed having them - i replaced them with Saulosi and miss them terribly
The tank looks great, except as Midas meantioned - its hard to make slate look natural. My only suggestion would be more rock - but as you get to know me you'll realise that in my world there can never be too much rock (a throw back from my demasoni days lol)
When I kept those guys i constantly tried to add a 3rd group of mbuna and it always ended up in a disaster. I dont think i would try it again.
Good job on the tank and keep us informed!