Aight, listen up, yo.....now before you startin hatin' check it out, aight...there should be no probs....they will deal with it...but check this...if you are runnin a tight setup that is locked down...and have say Tanganyika's then no. they are finicky and weak. i wouldnt mix...but if you have some haps and want to do it I say go for it. theyre both mostly carnivorous by nature and can both throw down oldskool when it comes to bouts....
but check this...if you have mbuna and want to mix in some JD's or firemouths or GT's or whatever then go for it. you'll be aight... but remember not only are PH usually higher for Malawi's @ around8.4 8.6 their intestinal tracts and digestive systems cant handle high protein diets that SA's crave. They get bloat real easy, dogg. As far as the water goes, most LFS sell fish that are all on the same filtration and its kept neutral because theres so many different species they hold....just use tap water that comes out and they will adjust. dont try and manage ph. you will cause more damage trying to keep it in check at a certain level because it may fluctuate from your own doings.... its easier to keep it tap,yo.
So check it....you'll get away with it. but you risk decreasing their life span. I think the nutrition and feeding habits are where the most probs are gonna cause you.....and just cause their happy, dont mean what your doing by housing them is right. mbuna are mainly omnivorous grazers and like brine shrimp as maybe 10-16 percent of their diet and should mostly be all the protein they get...the rest should be from plankton, krill, lettuces and maybe spirulina flake.....or spirulina enriched food.....also pellets are tight somemtes too...
just keep it real, and your fish will be locked down...know what I'm sayin' dogg....
peace