Here's the deal: to properly use any 6-7-8 speed DA rear derailleur, you must use a corresponding DA 6-7-8 speed shifter. Yes, there's that famous Sheldon work-around, but note my use of the word 'proper' above. With DT shifters, you can mix and match RD's and shifters within 6-7-8 speed parts, but if using 8 speed DA STI's, the last iteration of RD (and 7410 FD) seem to work best. If using bar end shifters, check the label on the indexing unit to determine compatibility. STI units aside, all of the fds can be used with friction, including later nine and ten speed models.

9 speed and 10 speed DA have the same cable pull as other Shimano products and aside from DA six seven eight speeders, can be mixed willy nilly with them as much as you choose to.