I got medium gauge strings on my F30R, which is one size up from yours. I bet the M120s or the "20" body size, is best suited for Light gauge strings, but really, it all depends on the neck joint and how solid you think it is. I know my F30 can handle it, it is so solidly built, but I am not sure I would do mediums on an F20 body shape. And if I do, I would cut the tension back a 1/2 step. Which would make it like light gauge tension, but still the big feel and volume of mediums. You can capo the first fret when playing with others.