I would have to agree with its not a good idea to mix the two. Not because of water conditions(PH or temp.), but because of how the different fish act towords each other. The mbunas are a very aggresive fish even towards fellow Lake Malawians(Peacocks and Haps), much less a fish that dosent even live on there continent. That green terror will be miserable until he is big enough to eat those mbunas, which he will when he's full grown. A green terror is similar to the oscar in that you buy a 55gal tank and you put a max of two in that tank and no more or nothing else.
But to awnser your question I think you should buy a many different mbunas(Auratus, Johanni, Albino Auratus, Cobalt Blue Zebra, Snow White Zebra, Kenyi, and Lemon Yellow to name a few) to briten up a tank and you will need to buy at least another 55gal to house these mbunas in, by themselves.