There really isn't much more to add to this, with what input already has been put out.
I wouldn't agree with the Africans not picky on the water chemistry, in THIS regard. Tangyangikans in particular have an even higher need for the higher hardness and electrolyte levels then even Malawi's and Victorians do, and they've evolved in this type of water over thousands of years. While tank raised stock may have grown in different waters, I think many hobbyists don't give quite the respect to this aspect as they should. I don't mean this in a derrogatory way, I'm just saying, that we as people have evolved in certain conditions over a very long period of time. Changes in enviromental conditions are usually gradual and not a very vast difference. I don't think there are any studies showing the impact of having species of any type of life living in conditions way outside of what they have evloved in, let alone the impact of doing as such. Beyond that, even if there is information of species outside of fish in general doing it, what possible harm is being asked of the hobbyist to simply keep the waters in the natural setting conditions as it is? If the problem lies that it means you can't mix certain fish you want to keep, maybe that's nature's way of suggesting you shouldn't mix such a mix.
I also do not like the idea of mixing SA's and CA's with Africans as many cases, the sheer size many SA and CA get to be full grown, usually doesn't make life good for the Africans. Sure something like say a Front might have comparable size, but I'd have to assume, such large mass fish like that competing for space and food, isn't going to be a good thing.
Diet is one thing I think you can actually control well. I don't want to sound like countering the valid point, but making use of pellets and certain types of flake foods can allow diet to not be as much of an issue. While many SA and CA eat more live and heavy protein type foods, they still will accept plant and vegetable matter based foods to some degree as well as your Tropheus need a certain amount of live and protein type foods too. It's a matter of not over doing it. Just as many hobbyists strictly adhere to no salt in catfish and scaleless fish type tanks, all fish still need a certain level of salt to maintain proper electrolytic balance. It's a touchy subject, and not directly relating to the topic at hand, but I just offer it for consideration.
