This is an interesting question. A lot of purists will condemn you for mixing fish from different lakes, and this seems to be well focused on African fishes. No one bats an eye if you mix, say, Rio Negro with Rio Orinoco (even though water chemistry is quite different), or even fish from northern Brazil with Uruguay, even though those are entire different climates!
Yet, mix Tanganyika with Malawi, oh boy, they're coming for you.
The water chemistry of the different lakes can play a component of whether you should mix them or not, but ... I don't believe in mucking up your tap water to suit the fishes anyhow. All too often, the fish wind up in a roller coaster of adjustments, and suffer more for it. The best solution is to pick fish that will go in your water, and our fish are incredibly adaptable. As long as extremes are avoided, including more sensitive fish, it makes no difference. Don't expect to keep and breed rams in malawi water.
But, if the fish are able to survive in your tap water, that's the chemistry you should keep them at (again, within reason). And, if all the fish are in the same water anyhow, what's it matter if you put a Frontosa in with an Oscar or a Peacock? As long as they get along and are cared for, who cares?