Originally Posted by
chemando
Are rear derailleurs rated for different speed cassetes? ie, will a Deore LX rear work with a 8speed cassette without issues?
Yes. Within a given manufacturer's models, the number of "speeds" is determined by the shifter so any of their rear derailleurs will work with any shifter/cassette combination. The shifters, cassette and chain must match but the rd is suitable for use with any.
Required disclaimer: Shimano Dura Ace pre 9-speed shifters, etc, aren't compatible with older stuff but that isn't a issue for you.