If you're going to give it a go, you might find a fair bit of refusal at the start just because formula tastes very different to breastmilk.
When we mixed fed DS I would BF him first and then offer a top up FF afterwards. You might want to let her have a little rest between breast and bottle to make sure she's eating to hunger and not be worried if she doesn't finish what you offer, which, to start with, could probably be as little as a couple of ounces.
We started mixed feeding at around 4m and I continued to BF until 12m, so you're almost there already
