The breeders explanation sounds very plausible! I know my puppies were always playing with each other and often would bite or scratch a little too hard. I had to try to protect the momma with a onesie, as she would get scratch marks around her teats as they would push to get her milk going long after they were eating well on their own. Mange usually starts with a loss of hair.
Mange in Dogs | PetMD This link might help reassure you.