Oh lordy thao, I don't think that guy should be working at a pet store...LOL if that were the case, all of mine with short legs would have long legs..
Anyway, the puppy food is better for them, it has the nutrients to help them grow and also more fat in it than the adult food..I think it's very important for them to be on puppy food for at least the first 6 mos. of their lives. Now, you can get canidae, and it's for all life stages..