You can use a Shimano 9 speed derailleur with a 10 speed shifter. In fact, Shimano shifters/derailleurs are pretty much intercompatible. The issue with Sram road and mtb gear, as has been previously stated, is that the cable pull is incompatible. Thus, you cannot use a Rival shifter with an mtb rear mech without fixing that, just as you can't use a Shimano 8 speed 600 derailleur with Dura Ace 8 speed.