Don't know if there is any magic to it or not. I have 2 community vieja tanks, with no aggression or injured fish. I have some fish that I have grown out, but also quite a few that I acquired at large sizes. I've had problems in the past trying to add zonatums to the mix, and one badass hartwegi (my current harwegi is harmless), so I am cautious of them... but mostly, no problems.
-Rich