Originally Posted by
3speed
^ The spacing is the same for 7/8/9sp, so the post 2^ is correct only in the case of those 3 speeds. Others aren't compatible no matter what shifters you use. You can't use a 10sp derailer to shift anything but a 10sp, even with the proper shifters. It is a brainless mechanism, yes, but people with brains built it and different speeds/companies often have different cable pull ratios that make them incompatible. As for the chain, I run a 9sp SRAM chain on my 8sp road bike. I don't Think it will be an issue. Either way, I'm pretty sure there will be no problem running an 8sp chain on the 9sp derailer, unless the clearances through the cage are Really tight, which I wouldn't think they would be on a MTB derailer(could be wrong).
Well, 10 speeds are a whole nother animal altogether, also, I guess I should have mention that many brands shifters and derailleurs (such as campy and shimano) can't be mixed. However, for the most part everything is pretty much interchangeable within a given brand. Shimano an SRAM are also interchangeable for the most part.