slate is good. limestone i think is a no no lol.
best thing todo is have some preppared or old tank water and test it for ph, gh.
prepare your other rocks in 1part bleach 10part water mix and soak them for a bit to remove any baddies that come with it. rise them and soak them in clean water and rinse them some more so all the bleach is well gone. then add them to the tank water and leave them over night, test for gh and ph again the next day, any changes?
also watch for rocks that continuesly bubble. this normaly indicates lime in the rock. discard any rocks that stain the water or bubble, or change the readings. i think + - .1Ph is ok and gh, you might even want it to raise your hardness. but allways test none the less.