9sp/10sp compatibility in Shimano SLX drivetrains
The difference in chain width between 9s and 10s is less than 1mm so it won't make much of a difference once the mech is adjusted correctly. I have M660 on my bike and M665 on brother's bike and they aren't so much different, so if 10s is proven to work on one, it should work on the other.
Front shifters have the same cable pull, but front derailleurs have different travel rate between 9s and 10s. Therefore if you want to use a 10s shifter with a 9s crank it's actually better to use a 9s derailleur. Although both would work, a 9s would be more accurate in this case.