It's best to look at it at a species by species basis, as there are many variables involved, even when mixing fish from the same lakes, and it's impossible to generalize. From tanganyika you could be thinking of the 14" long carnivorous Frontosa, or perhaps a 1" Carnivorous Multifasciatus, or do you mean the little herbivorous gobie or Tropheus, etc. etc.
If we're going with generalization then the safe answer is that it's best not to mix Tanganyikans with Malawis as they have very different behavioral and often diet traits and even if it works, that doesn't mean we're doing the best thing for them. Victorian Haps can, however, be safely mixed with Malawi Haps of similar temperment.