The Bigger claws are used for defense but i have seen them pick stuff up with them as well. the little claws underneath are only used to grab food with and burrow and have no real pinch to them whatsoever. if the fish is fast enough the crays will never catch them. i have a 40g tall hex tank with a blue cray and tons of guppies. very rarely does the cray ever catch one and if it does i never notice cause of how fast they breed. It doesnt matter though how much its been fed if the cray can kill a fish it will just to do so.
Hope that helps,