Originally Posted by
Lemond1985
All Shimano rear derailleurs that are 6, 7, 8, 9, or 10 speed have the same amount of pull, and so are interchangeable. So there is no such thing as, for example, a "7 speed Shimano rear derailleur" all of them pull the same amount of cable per shift. There is a rare proprietary 8 speed Dura Ace shifter/rear derailluer combination, but I have never run into it.
Are they not designed for different chain widths? All of my bikes are 7-speed (so I have parts interchangeability) and I don't think the 9-10 speed derailleurs will accommodate the width of a 7-speed chain.