whatever you use the wax you already have on must be stripped off or just waxing old wax. You can do this by buffing with an extremely light compound or wax remover from the detailing section of your automotive store.
BTS (bow to stern) will do it all. it removes old wax and protects the gelcoat. can also use it on the entire boat, trailer, cowl, seats, dash, trolling motor, etc.
Sure, all of them do. All of these are a cleaner/wax/protection to paint as well as fiberglass. There are some specially made for fiberglass but for protection and cleaning fiberglass acts a lot like paint.