As is 6700, and while both 6600 and 6700 (and any other 10 speed Shimano road shifters) will work with a 9 speed MTB mech, as the cable pull ratios are the same, neither will work with a 10 speed MTB mech, which has a different pull ratio.
As is 6700, and while both 6600 and 6700 (and any other 10 speed Shimano road shifters) will work with a 9 speed MTB mech, as the cable pull ratios are the same, neither will work with a 10 speed MTB mech, which has a different pull ratio.
6600 is 10sp.