Combining has little to do with ignorance, but instead, personal preference, and research.
The largest wrenches in keeping S Americans, with N Americans, are water parameters.
Central (north) Americans like Vieja, have evolved to live in high pH, mineral rich water parameters.
South Americans like severums and Cichla, have evolved to live in the soft, low pH water parameters.
Problems of keeping them together can often be mitigated with lots of large water changes, that hold nitrate to a non-stressful level (for me, < 5ppm).
View attachment 1518596
Above Central American water parameters where Vieja are found.
Below severum habitat.
Almost the difference between fresh, and ocean water.
View attachment 1518597