Y.M.
I would not mix formula and breastmilk, I would offer the formula and see what happens. I think there will be more issues with the delivery system then with the formula...LOL.
I had both of my little ones on Soy Formulas, because their dad is lactose intolerant and the had spit up and gas problems with regular formula. You might also aspediatrician if your baby can switch to Milk, they usually start at 12 months on whole milk. Or PediaSure is a total hit with added vitamins and nutrients and it is flavored.
Hope this helps.