The braces won't interfere with the approach to a chin reduction, and the surgery won't interfere with the braces. There are several ways to reduce the chin prominence depending on how prominent your chin is. Have your surgeon show you the possible changes from the different procedures on the computer ahead of time.
Chin reduction is not as simple as chin augmentation. As long as you have braces on, the lips have more prominence so I would wait until they are off before attempting anything. I would also make sure your doctor has done this often and can show you lots of pictures of their own patients with nice results.