Wait - I am confused, do you want a SA, softer, acidic water tank or an African, harder water tank? You could go with kribensis, as they don't have particular requirements either way, and are riverine Africans. They will spawn for you pretty well if you provide some caves, maybe with a broken flower pot or two.
If you are going SA, then you could certainly go with the old standby, convicts. They stay relatively small and will spawn like crazy. These too have been tank bred for so long that I doubt you would have trouble with them no matter what kind of water you have. Yellow labs (labidochromis caeruleus) would be nice and they, too, have been tank bred for such a while now that if your
LFS keep them without adjusting their water then you can, too. Ditto on the
UGF - not necessary and another thing to maintain. Lots of folks still use them, but I gave them up years ago and never went back.
Slate is inert, and granite should be too, but you need to watch for heavy metals in rocks you collect, which would be evidenced by a red streak (iron) through it in any area on the rock. Round river rocks are generally inert, as well.