Slate won't affect pH at all. I put some big pieces of malaysian and Mopani wood in my tank, and several large chunks of coral to buffer the pH. Took a few months and loads of carbon for the wood to stop turning the water dark. Stock is 3 red and 1 green severum, 5 silver dollars almost as big as the severums, 9 Colombian and 10 white (black) tetras, 8-9 kubotai botia's, about the same number of BN pleco's, a single Synodontis eupterus, a Macana zebra knife fish, and 4 recently added electric yellow Lab's. Pretty peaceful tank for the most part, though I'm keeping an eye on the green severum as it grows larger. The 3 reds (1m/2f, lost the other male last year) are bigger/older, and 2 seem to be more-or-less paired up but haven't spawned yet.
All the plants are plastic, the severums and silver dollars eat anything real. I occasionally toss in excess Salvinia and duckweed from other tanks.