Originally Posted by
gerv
Rear derailleurs don't care how many speeds you have on the cassette. I have the same Deore derailleur model on both my 9 and 7 speed bikes.
Also, Alivio, Altus, Deore...it doesn't matter. The derailleur *size* matters as it relates to chain capacity and cassette size (of the largest cog). Within Shimano, you should be OK.