FX USA told me a couple weeks ago that there is no bottom limit to regulator setting as far as damage to the gun goes, but that below 80 the regulator may not function as well. I think that warning refers to shooting after your bottle pressure is below the reg setting, which is itself no big deal but if you get very low you could damage the valve because there is so little pressure resisting the hammer that the valve slams forward too hard.
Otherwise, for low reg settings you just decrease hammer by the wheel or preload screw or both and you're OK. IOW, when you decrease pressure you should decrease hammer. I think there is a little translation error in the manual giving the impression that the factory setting is the "official" one, when what they mean is that once you get much below the set regulator pressure your hammer may be too much.