I'd like to ask if a Shimano XTR M980 Shadow 10 Speed Rear Mech is compitable with a 9 speed system (Cassette,chain,shifters);
No it is not compatible with 9 speed systems. You will need the M970 9 speed RD.
If only there was a "sticky" for this very subject...
Rollin With No Colon!
The Bike Whisperer
Crohn's and Colitis Foundation of America