I have seen carp hit shoals of Roach fry up here in Scotland. I have also seen them go mad over tadpoles/baby frogs, and on one occasion, landed a 14 pounder on a half herring pike bait!
I know of fellow lure anglers who have caught them on various spinners etc, and the Americans diliberatly fish for them using jigs and shad baits.
I think a lot depends on the water, if its rich in natural food, then i think predation is not so apparent, but, a Carp will not pass up an easy food source. I think a lot of out fish do feed on others, its just that we never really know because of the methods we fish (Yanks use lures far more than us)