Light rust will be scrubbed off after several applications. We get rust almost overnight with some brands of vehicles. Who said that you needed new rotors and why? Ask to see the parts that must be replaced or ask for the old parts next time. There is an abundance of charlatans and less than reasonably intelligent mechanics in the auto repair business. Light or heavy braking should not matter; just brake to remove rust.
Rust, can progress, however, so that the swept braking area on a rotor is diminished. Even so, brakes can work quite well according to my experience with some reduction in swept area but don’t let this go too long.