The most common problem with mixing South and Central American cichlids together, is that the water parameters are so different.
The average pH of the Amazon river is 6.7, and water is soft, and in some river a pH of below 5 is not uncommon (Rio Negro, and Rio Oriniico are examples
The average pH in Central American waters is 8 and water is hard. in some area, up tp pH 9.
The difference is over 100 fold
And those differences manifest themselves in a fises resistance to disease, and immunity to bacteria.
A fish that has evolved over millenia in soft low pH water will resist diseases common to that water.
Put that same fish into hard high pH water, and while it may not, or won't immediately go belly up....
over time, it can become infected with debilitating diseases such as HITH or bloat and become scarred up, and live an unhealthy life.
And/or vica versa.
With certain short lived fish like many aquarium tetras, or live bearers it won't be problematic because many don't live past 3 years of age anyway.
But many cichlid will easily make it 8 to 10 years, and those debilitating factors don't start to manifest util they are 2 or 3 yoa.
Another factor not taken into consideration by many aquarists, is that in S America many cichlids come from large mixed cichlid communities, and have evolved to share them.
In Central America there are far few cichlid species, so many are loners, where 1 or 2 species may share an entire river,
and if territories overlap, come into conflict.
There are similar disparities between those cichlids from east and west of the Andes.
Cichlids from the Amazon often live in large varied communities, but west of the Andes....
where may be only 2 species of cichlid that share and entire riverine habitat (mush like in Central America) care must be taken to provide territorial them with appropriate space.