Plan out how long you expect to age the beer with brett and add the fruit during the last 6-8 weeks.
If you aren't sure how long you want to age the beer then pitch brett and once you're happy with the flavor then add the fruit for 6-8 weeks.
I'm not concerned with brett's ability to chew through whatever you throw at it but sitting on the fruit for longer just means you're losing some of the freshness of the fruit. FWIW I would look for heirloom or other particularly flavorful peaches for the beer. My experience with the normal grocery store varieties has been underwhelming.
Thanks for this advice. I am going to put the beer in secondary w/ the brett, I bet it wont be too long to age on Brett. Then adding peaches later, I have access to good varieties I've frozen from my parents trees.