I would not recommend doing different surgery while you are planning/waiting for your jaw surgery, it will add scar and problems.
By the time you finish your orthodontic work in preparation for jaw surgery you will be ready for you final surgery.
Ali Totonchi M.D.
Chin Implant Before Orthognathic Surgery
Placing a chin implant now is certainly possible with its removal later. At the time of your orthognathic surgery at age 21, the implant can be removed if the forward jaw movement is enough or removed and replaced by a new implant or a sliding genioplasty.