Garlic being of the same family as onions has the same thiosulfate (e.g. sodium 2-propenyl thiosulfate) which has the potential to induce hemolytic anemia in dogs. In particular this involves oxidative damage to the erythrocytes causing their lysis.

There are no existing scientific studies to indicate garlic has any effect on fleas. Most likely the idea was that the components of ingested garlic could somehow ooze back out of the pours of the skin creating a smell to repell the fleas. Or that the blood would be tinted with a taste unpleasant to fleas. But then again a hungry female flea isn't too picky per taste. Also if the concentration is high enough for the secondary metabolites to come out of the skin pours then you probably have a high enough concentration to worry about hemolytic anemia.

A companion substance often mentioned with garlic for fleas is brewers yeast. There is a study out on the effect of brewer's yeast, and at 14 gms/day it failed to repell or kill fleas. Time to stick to the Proban containing line of products (e.g. Sentinel)

I would not hesitate to give my dogs turkey skin - low fat high protein at the upcoming holiday. My two chow hound puppies would probably want more turkey skin than the turkey has to offer.

If your dog has a history of pancreatitis then of course you need to avoid anything out of the ordinary for feeding.