I am a practical purist, if something needs to be moved from one tank to another, and it means mixing one continent with another I'll do it to stave off aggression, but must admit when I see old world fish mixed with new worlds, it always seems to to appear a bit goofy, and I try not to.
But I not only think new and old appears goofy, mixing many Central Americans with S Americans appears somewhat strange to me, there aggression levels are often very different, and needs quite different.
And mixing 2 of the same genus is often even more strange to me.
In nature it is very rare to find 2 species of Parachromis or 2 species of Vieja in the same habitat, unless man has introduced (managuense has been introduced all over Mexico and Central America where it is not normally found) or inadvertently combined them by connecting waterways, such as in the Panama canal. An example of such man induced mixing in that area, may be xC (Vieja) tuyrense and Vieja maculacauda.