As the proud of of a 7in long crayfish, I wouldn't suggest them as predators, because they're not the most successful thing out there. They're primarily scavengers, and snagging a live fish is rarity. Only saw it happen once, he attacked the pleco (which got away on it's own). The pleco is now in a different tank, incedentally.
That being said, he did eat a grown male guppy a few days ago, based on its disappearance. But I still have a male and 10 babies in there which he doesn't actively hunt, he just waits for one to come near.