Hmm . . . I don't know specifically about pomogranate seeds or celery, but in general its a good idea to let a cat eat only the cat food he is accustomed to.
The reason is threefold.
1. Cats have very sensitive pH balances in their digestive systems. Too much of a food that is not balanced for cats, is a cheap food, or when a cat's diet abruptly changes, the pH balance is thrown off, evnetually causing crystals in the bladder, which leads to UTI's, bladder crystals, and urinary blockages, even bladder stones.
2. Some foods which are fine for humans are toxic to other animals in large amounts. Although you might not notice a change in the animal (say when you feed grapes every once in awhile to a dog), although no change is detected, the toxins build up and damage the liver and kidneys slowly. This is of course not true of most foods, but you always want to be careful; you'd be surprised what weird things cats and dogs have natural intolerances to.
3. In general, feeding certain types of human foods may offset the natural health balance of his diet, cause digestive issues, diarrhea or constipation, and on top of that, your cat may have an allergy to something that you'd never even realize. These are all only possibilities, but I'd do some research. There are many types of human foods, meats and vegetables, which are healthy for cats which your cat might love.
9.6 million euthanized in US shelters every year.
It doesn't have to be this way . . .
Please spay and neuter.
Don't breed or buy while shelter pets die.