Normal doesn't mean standard. The Yorkshire Terrier Club of America has a standard for Yorkies, and requires that it's breeders always breed to standard, this doesn't mean that all puppies will meet standard, but they should be rather close. Those dogs who don't meet standard should not be bred, but they make perfectly adorable pets. Unfortunately many breeders are just in it for the money, and aren't actually trying to improve the breed. I hope you give serious thought before becoming a breeder, and familiarize yourself with all the problems a yorkie can have. I'm actually surprised the rescue didn't require sterilization, usually they do. Here's a link to the YTCA website, and it's definition of standard.
Yorkshire Terrier Club of America. Welcome to YorkieTalk!