I have some Red-claw crabs (the only FW crabs I've seen in fish stores locally), and they do fine. I have tall plants that go above the water, but I've never seen the crabs climb them to get air access (figured they would, but I've never seen them). And they don't bother the fish, they're more afraid of them. And they don't bother snails at all, in fact they usually hide together.
The only problem is that they hide all the time! Only rare-ly I see one walking around, and I think my Pictus Catfish is scaring them too much.
Neat little guys, they're awesome, i just never see them. I've only had them a few weeks, maybe it just takes some time for them to get used to the tank?
I also add Iodine suppliment to the tank every week. Its sold as a SW additive. I also added it for the Ghost shrimp, so they can molt properly.