OK - I hear what you say: You want to fix the problem with as little "overkill" as possible.
I do not have any experience that would lead me to believe that using a "heavy oxidation" product on actual "light oxidation" would cause problems by being overly harsh to the hull's gelcoat. I am not sure whether this is a problem or not. Hopefully, others will be along shortly with more insight than I have.
As to your stains on the waterline: I have a couple of suggestions on products to use if the one you purchased does not do the trick to your satisfaction. Over the years, I have had success with a product called "FSR" - stands for Fiberglass Stain Remover, and acetone.
If you use acetone, you need to be careful because it will also remove a little bottom paint and give you a little color "smear" on the fiberglass that you then have to clean off (using more acetone, which can cause "smears"... and around you go). When I use acetone sometimes (if I remember) I will cover the edge of the bottom paint with some masking tape to prevent the "smears".
Although the traditionalist are going to start screaming, my solution to waterline stains and waterline slime and barnacle growth is to raise the height of the bottom paint another 2 - 3 inches (or whatever it takes to cover the area under discussion). The stains may still be there, but they do not show up on the bottom paint the way they do on a white hull. If you decide to go the route of raising the bottom paint, you will need to figure out whether your boot strip is within the gelcoat, or is an added on vinyl graphic. If gelcoat, go ahead and paint away. If vinyl graphic, you will need to remove it first - do not paint over vinyl graphics.
Keep in mind the fastest way to the mooring is paint the bottom and launch it! You cannot see any hull oxidation or waterline scum marks from the cockpit!
If you do find a good cleaner or wax or combo product that does the trick for you, let us what worked for you.
Happy Sailing.
