I have been exclusively pumping since my baby was born in November (he wouldn't latch). Now he is drinking more than I can provide. He drinks about 26 oz a day and I only can pump about 21 oz. I have a good freezer stash but I am about to start buying formula to help me supplement. Should I start by mixing breastmilk with the formula? Do I need different bottles? I currently use Medela. Sorry, these seem like dumb questions but I don't know anything about formula! Thanks!