I put my pitiful amount of my milk in my sons bottles all the time. It's usually less than an oz. i keep it in the fridge until I pour in his prepared formula. The bm is good for 6 hours but formula only 4 hrs... So if my son doesn't finish the bottle within 4 hours yeah it's a waste but he usually does!!
I've heard that it was best to give the baby the breast milk first then the formula to avoid the breast milk from going to waste in case baby doesn't finish the entire bottle. Your choice though. @eiliahelocin