SRAM rear derailleurs use a 1:1 acuation ratio, meaning that 1mm of cable movement causes 1mm of derailleur movement. Shimano uses a 2:1 ratio, so Shimano and SRAM rear derailleurs are not interchangeable.
From basic bike repairs to quality bike upgrades, we have a good range of Shimano rear derailleurs available both on-line and in-store.
A Shimano rear derailleur designed for a given number of gears can also be used on drivetrains with less gears. This applies from 6 speed through to 9 speed drivetrains (and 10-speed road). So a 6 speed drivetrain is compatible with a 6, 7, 8 or 9 speed rear derailleur. A 7-speed drivetrain with a 7, 8 or 9 speed rear derailleur etc. The exceptions to this interchangeability are 5-speed drivetrains (now rare) and Dyna-Sys (Mountain Bike) 10-speed drivetrains. The Dyna-Sys rear derailleurs are not interchangeable with any others.