If he's not moving when he should be startled, I'd be worried, but if not, he's likely just waiting for a meal. Most of the lobster-things I've seen don't move around much. I'd give him the log back; they like to hide.

It's not a good idea to keep them with fish since they can easily sneak up on sleeping fish and eat them. It doesn't always happen, but it can. I would think that keeping him well-fed would reduce the risk.

When you say that your water is fine for him, do you mean just pH, hardness, and temperature, or is your tank also cycled? If it's not cycled, it could be stressing him.