A.M.
My opinion on this is that if it's not broke...don't try to fix it. But if you are looking to save money by not having to provide formula, you could try to mix them little by little. Again though, I would stick to what is already working for you.
On a side note, I just wanted to point out that in my state, those who are on WIC are only allowed to buy Good Start formula. They have to have a dr.'s note to get anything else. I can remember thinking about using Good Start and finding that it was never in stock. When I asked the store about this, they explained about WIC and that's why they had trouble stocking it. So yes...that "mother" had a point. :) Good luck!