I have never seen kuhliis or cories dig in gravel. It just doesn't happen, any gravel is too big for them. I recently got a new tank and because I'm going to have kuhliis in it, I put fine sand (about 0.1-0.6 mm in diameter) on the bottom. A bottom substrate with a slightly larger diameter than that works too, but anything classified as gravel is too large IMO.
Other than that, I think Inchworm gave very good advice.