Personally, I wouldn't hesitate to use sandpaper on the braking surface if it has surface rust. Hand sanding will do it.
Use something like 150 grit to bite into the surface rust then 180 to shine it up.
As long as you don't go mad in any one spot it'll be ok.
Would be better done with discs off, and you could give the pads a quick scuff at the same time.
After driving a while, everything should bed in nicely.
Nothing lost in trying it apart from time.
***If the corrosion has really bitten into the disc then they really need replacing or skimming.***
If they can groove and drill a brake disc, a few scratches won't kill it.
Discs, drums, flywheels ect are tougher than you think. Try biting one.