How and when to supplement

alge44

DD is just over 6 months old. She's been exclusively breastfed, but I'm having trouble keeping up with pumping enough now that she's up to 5 oz per bottle. As much as I don't want to, I think it's time to start supplementing or burn through my freezer stash. So my questions: 

 Is it better to go through my freezer stash and do just breastmilk for another month then start formula at 7 months or start adding formula now?

When we do start formula, is it better to mix milk and formula or do 2 milk bottles and one formula bottle?

Thanks in advance!