I've just put 2 mountain shrimp in my big tank and they seem to be getting along fine. It's only been a couple of days though but I'll post an update if anyone's interested.
They've camped on a large piece of wood which is in the flow from the filter outlet.
My catfish haven't paid any attention to them yet, the Angelfish were curious when the bag was floating in the tank but soon swam off when they saw the size of them.
I asked my lfs about them, they said they are not big polluters and should clean up any scraps of food that the others miss.