Shih Tzus are also prone to gain weight - so mine get 1/2 cup of food per day (except nursing mommas - who get quite a bit more). I also use a super premium food (Chicken Soup for the Dog Lover's Soul until recently, when I put everyone on Flint River Ranch).
I too agree that a puppy food isn't absolutely necessary. My vet said that the recommendations for puppy food for one year were made 30 years or so ago when dog food wasn't nearly the high quality it is today. Now with the human quality food ingredients and the higher protein levels, most adult super premium foods are fine.