If the cat become frightened, increase the distance between the animals and progress more slowly. Eventually, the animals should be brought close enough together to allow them to investigate each other. You may want to keep your dog on a leash and with you when the cat is free in the house during the introduction process. Be sure that your cat has an escape route and a place to hide. Keep the dog and cat separated when you aren't home until you are certain they will both be safe.
Dogs like to eat cat food because it is very high in protein and therefore very tasty.
Why dogs like to eat cat feces is not well understood but is a relatively common behavior. Although there are no health hazards to the dog from this habit, it is usually distasteful to owners. Attempts to keep the dog out of the litter box by boobytrapping will also keep the cat away as well. Punishment after the fact will not change the dog's behavior. Always feed your dog alone.