Okay....just to play Devil's advocate for a second here.....let's assume that there weren't ANY aggression issues at all. If they all seem to be happy, healthy and safe....does it matter that they aren't from the same geologic region or body of water? I admit i'm not at all sure about water parameters for any of them, but we all know that fish can and will adapt to a wide margin of "ideal" water parameters(even if they aren't exactly what the fish needs). Again...playing DA.
Removing the overstocking part of the equation, is the problem that the general fishkeeping population has with these types of set-ups the fact that they "shouldn't " be doing this type of thing, or that it's a major harm to the fish? I've often wondered the same types of questions about many other species as well. I'm not trying to be an a$$, i'm just trying to learn more.
Please don't tell me to read more either..LOL
Tanganyika(sp.), African, Victoria, South American, etc., etc., etc....
If they are similar and no ones getting hurt or killed or stuck in a corner...why not mix em' up?
I'm sure i'll be dragged over the coals for this...okay, let me have the input folks. It makes for good education and experience.