I'm not absolutely sure about this, but in my experience just 2 tribbles eat (and breed) per hour. So to build your carcass grinder, you need 2 normal tribbles, a stack of food and 1 cannibal in that order. This worked for me. So one of the normal tribbles will reproduce and one of the three will be eaten by the cannibal. Theoretically this would work with just 1 normal tribble too, but if the cannibal eats first, the chain is stopped.