Yup, not a matter of if but when the cray will get one of your fish. It will be at night when the fish are resting and vulnerable. I have no idea what you're saying, that the pleco is on a shelf so the cray can't get to it, I just cannot imagine that being possible - I had a crayfish that could get ANYWHERE, he would literally climb all the way up tall skinny plants, or all the way up the filter intake or heater, trying to get out. I was amazed by his climbing abilities, since I also thought at first that he'd stay on or close to the bottom. But it turned out, he enjoyed hanging from various aquarium decor that you'd never think he could get up on.
Also, what kind of cray? As with the pleco, any cray I can think of besides maybe a dwarf cray needs at least 20-30G tank.