Q: how thin can rotors be before replacing?

Nothing sounds right about that. There are references that tell mechanics how thick rotors are when new, when they’re too thin to machine, and when to discard. The rotors need measured with a micrometer to check this. Rust and some other factors can shorten the life of a rotor before it’s too thin, but you should have been informed of this. They are many reasons to replace a caliper. Brake pads being too thick are not one of them. Either the piston in the caliper is not compressing all the way, there’s a seized caliper slide, or they have the wrong pads. If the piston is stuck, this warrants replacement. I would get a second opinion from a certified mechanic, such as one from YourMechanic, who can diagnose your brake system and suggest what to do from there. Good luck.