Nothing wrong with sandstone. Slate has a high chance of containing iron oxide (rust) and should be used with caution.
Trust me i was a rock hound/stone mason
As far as the paversand is concerned as long as there is no additives, in example for mold prevention, it will be fine. Rumor has it that some of the paversands contain aragonite so it can act as a buffer giving u the water parameters of african cichlid tanks....