I think the only substrate you should use right now, considering the size of the gecko, is paper towel. If you use a bark or other type of bedding, he may have a tad bit of difficulty catching his live food items, which tend to run around on, and at times burrow in the ground. Also, Cresteds are rather enthusiastic eaters, and will likely get things in their mouths aside from their food.
The paper towel should keep enough moisture for them, after all, they don't need a heck of a lot. A good spray in the morning and another small spray in the evening should be just fine.
Cresteds like to crawl around on flat surfaces, so branches don't tend to appeal to them. If you're looking to put something else in there, I would recomend getting a piece of corkbark.
He looks like he'll be just fine in there. But make sure to get some meat in him, he looks a tad small :)
Have Fun
