HCA: Hermit Crab Association

I'm not sure about the gravel, or the moss. I'm not the most experienced with topics like this.
I would wait for others to post, before you add it.

For the wood, you just need to find what kind of wood it is, and see if it's safe.

I agree with above, I would change them everyday, completely. Use Prime conditioner.
It's an easy, and well recommended conditioner.

Change the bowls as soon as you can if they drag substrate in the pools.