Once a Yorkie is 4 pounds, they should be able to regulate their blood sugar. Joey never needed Nutri-cal, it's best to know the symptoms of hypoglycemia, and treat with Nutri-cal or Karo syrup if they are experiencing low blood sugar. You know how you give a piece of candy or a glass of orange juice to a diabetic if they have an attack of low blood sugar, this is the same thing. Nutri-cal is mainly sugar (corn syrup) and you don't want to give it to "prevent" hypoglycemia, but to treat it. Protein should always be given within 20 minutes. Too much Nutri-cal can cause an attack of low blood sugar, so be careful how you use it. Here's some information on the symptoms of hypoglycemia, or low blood sugar.
Toy Breed Hypoglycemia