There's plenty of people who do that, whether they plan to or not. I don't see any downside, any BFing you can do will give LO extra antibodies and good stuff, even if it's just for a few weeks. And you'll save $ by going a little while without formula.
It's not uncommon! It's great your LO will get the benefits of BF for a few months. Do some research on how to dry up your supply gradually to prevent pain/mastitis/plugged ducts. You may be able to keep nursing in the morning and evening and give formula during the day, if you want.
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Cons: The Talkers of the world. Do what you want to do and don't let people make you feel bad for your decisions.