A comment like that from a breeder would make me look for another breeder. The food you feed does not determine adult size.
Breeders and pet parents here have Yorkies of all sizes in their homes that get along great. I had the same question you did when I was looking for my second. Ultimately I decided against a little one and went with one that is closer in size. I also chose not to get a puppy -- Teddy was almost 8 months when we brought him home, and he was still very puppy like.
There is no magic formula for size or age differences. It really comes down to the individual pups. Some matches are fantastic, and some are more of a challenge.
Best wishes in adding your second.