The wife and I have discussed keeping crabs as well, and from the quick research I just conducted, you can rule out the rainbow crabs. Here are a couple of quotes from Practical Fishkeeping's article about them:
"Size: Adults can reach up to about 15-20cm/6-8" across the carapace, although most captive specimens remain considerably smaller.
Habitat: Research papers cite the occurence of Cardiosoma armatum on sand and mud flats on the lower Volta river system in Africa. The species tends to burrow and produce a hide-out in the mud. The species is caught in empty tins. It does not need very much water and is happy with either access to freshwater or slightly brackish water.
Aquarium: Since this is primarily a "land crab" it should be given access to a land area in the aquarium. It is not well suited to life underwater and will definitely try to climb out if you don't provide it with somewhere to sit above the water level.
Diet: Most crabs are opportunists, and this one is no exception it seems. It accepts most foods, almost anything, in fact, but also has a tendency to try and eat fish. It wouldn't be wise to house it alongside fish. What it can't catch it could injure with its hefty, sharp pincers."
So in summary, at 6-8" potential size, and a preference for a terrestrial environment, I think you can rule this one out.
I was thinking about moon/halloween crabs myself, but again they are terrestrial. I think in the end, I'll be left with plain old fiddlers.
