I did this with my daughter, she had breast and bottle from day 2 of her life. We used pumped milk and soy formula, once we figured out which formula she liked and didn't give her a belly ache it was fine. We did eventually switch to just bottle/formula at 4 months because I was taking medication and worried about how it could affect the baby. Otherwise we had no issues switching between before then with her. She's now two and in great health. I highly suggest a breast-like nippled bottle and one with vented air flow, or the drop in bags (greatly reduced gas and promoted continuous sucking so the baby doesn't get frustrated).
