Sooner or later the Jack Dempsey will grow to 9" - 10" inches. He will become territorial and will start killing and eating the smaller Africans. The second scenario; The smaller, quicker and quite aggressive Mbuna will constantly punch and run from the JD, picking his fins apart, making his life miserable.
You might be able to strike a balance between Africans and New Worlds for a while, but that will change over time. At that future point adjustments will have to be made.
I've mixed Africans and New Worlds in the past and had relatively good success, but it was awkward. I rehomed most of my africans, and traded some in at my LFS.