I have 3 different species in my 90 gal. So you be able to.
Breeding group of
Cynotilapia sp hara gallireya reef
Pseudotropheus polit
Pseudotropheus msobo
Should you mix the two Pseudotropheus? Can't that be problematic? Possible crossbreeding?
That is a good generalization to make. Erring on the side of caution is better than being careless, but it's not a hard and fast rule in all cases.
Much of the scientific research I've come across on cichlid mating triggers and their evolution highly supports that the female is the decision maker. She's typically looking for certain triggers such as body shape, coloration, patterns, etc.
For the particular species listed, the males look quite different from one another. The females are different as well to some degree. One should still be careful, but the risk factor for that particular combination is much less than a tank with Ps. msobo, Ps. saulosi, and Ps. demasoni (all males with a similar body shape and dark coloration on a blue background, with orange females (less the demasoni females).