I would not recommend Pro Plan at all. Too many controversial ingredients and Purina is not a reputable company. Take a gander at this website:
Dog Food Analysis - Reviews of kibble and try to get a 4-5 star food. 6 star foods are VERY high in protein and some yorkies do fine on them and others not so much. I personally feed Acana Grainfree Grasslands & Prairie formulas.
Good luck!!
Vivian