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I wouldn't advise it as your entire substrate, but it probably wouldn't hurt to have a little area be a waterfall with rocks at the bottom, as long as they're too large to easily eat. I might go a bit larger than 1 cm, though. And make sure they're smooth and rounded, so if your crestie *does* try to eat one, it is more likely to pass through without difficulty...

I wouldn't if i were you. It would be much safer to have moss or paper towels when it is young. Also if you put in too much water then it could raise the humidity up too high.