I would say give the dems another chance. was the dem the same size? if so, he may have the advantage.... But, they stay smaller than most all other mbuna. So, if you have larger mbuna with the dem, you prob wont have any issues...I have red zebra, msobo, albino socolofi and they allget along just fine with the dems. my juvie msobo dont get picked on by the dems. My point is, the dems dont pay much attention to the larger guys...
So, either find a dem that is smaller than the tankmates right now, or wait till your current stock is older and get a bigger one..