Is it ok to feed while baby is on formula?

I was told that when you switch a baby to formula (I'm getting ready to go back to work in a month) that you can still feed your baby, either in the mornings, at night, or still pump and feed the baby milk through a bottle, and offer formula in between feedings (as a supplement). I have also heard that if you switch to formula a baby may choose to eat one or the other and then you are stuck with whatever the baby has chosen to eat... I'm going back to work after 12 weeks, but I would like to continue feeding up to 6 months but didn't know if that was possible... any advice is appreciated! Thanks!