Ash, Im sure you dont have the evidence, because your not stupid enough to experiment like that.
Yes, if somebody is new to fishkeeping, then you expect them to have a few incompatible fish, or not quite perfect water conditions, because it is all about learning.
But when people say 'I know I shouldnt, but...'
Well, these people are in the wrong hobby.
Yes, Malawis will eat live foods, frozen foods, flakes, lettuce, infact Malawis will eat anything offered to them... thats because they are opportunistic feeders.
However, in the main Malawis eat vegetation, and if they have to much in the way of protein, bloat sets in.
There is simply no excuse for an experianced hobbiest to keep incompatible fish together, what ever reason you try to give.