I had FUE transplant on scar tissue, what is the success rate on scar tissue? (Photo)

Hair transplants don't take as well in scar tissue as they do in regular skin, though often the scar tissue becomes healthier after transplantation.  A second transplantation may be desired once things heal.Safe use of peroxide would depend on how recent your surgery was; that question may be better addressed with your surgeon.

A hair transplant in a scar usually works well and is commonly done in facial beard and mustache as you seem to have done