Squado, I don't know about the Emerald Crab at all, but the cleaner shrimp would be sorta hit and miss, in my opinion. You might be able to tell the docility of your CBS by watching it and seeing how much of a meanie it acts like (snapping at other fish, etc). Mine was right mean, so I didn't even chance other shrimp with it, in my 29 gallon tank.
trotty, IMO you should be fine, IF the CBS are a true pair. Make sure the source of the shrimp can verify that they are and aren't just trying to sell you two shrimp. I think they are better as having only one of a species in the tank. Else they'll fight each other. But with lots of space like that you 'should' be okay.