I have 2 crabs currently in a 20g tall, but I just picked up a 10g for free so I'll probably be swapping them to that so I can convert the 20 back to a community tank. They won't be losing much floor space and it will give me a chance to make a better environment now that I've learned more about their care.
My crabs prefer sand over the coconut fiber I first used. They love digging in it. They'll even take water into their shell and release it over the sand to make it easier to dig.
I've never thought about using plants in my crabitat partly because some plants can be toxic, but mostly because they are so destructive.
Edit: Ah, I just realized that you were talking about RCC, not PCC hermits. Oops...