I can't say no enough times to express just how much of a big no no red claw crabs are with fish...
Here are a few facts though...
They eat fish... do NOT listen or trust the fish shop when they say they don't
They need access to land at all times as they are not fully aquatic
They are actually brackish (I think... pretty sure they are)
Basically if you like them, get one or two, but give them their own special tank. Don't even try to mix them with fish.
Do a bit of googling and you should find some interesting information on how to keep them if you ever wanted to give them there own species only tank.

That is pretty cute... but not entirely practical. There is a very clear drawback that it'll never have fresh air unless you remove and replace it.
Edit: Or maybe not? Having read the blurb on that site it suggests you attach an airline too it... in which case it's a pretty awesome idea. And would allow you to mix crabs with fish.
But still stricly no bottom or mid water dwellers. I'd only mix them with fish that will definitely always be above the reach of the crabs.