Originally Posted by
dunkleosteus
1. The number of speeds of a derailleur is an upper limit of the number of speeds it can actually run, like a 10 speed RD would be fine shifting across a 7 speed cassette, and so on.
Assuming the same cable pull (indexed shifting) or friction shifting. I did run into a problem setting up a Campagnolo Mirage 9-speed derailleur on my wife's 5-sprocket cluster. Even with the lower limit adjuster fully screwed in, it couldn't limit the travel of the derailleur enough to keep it out of the spokes. I had to replace the limit adjuster with a longer bolt to make it work.
2. The "number of speeds" of an indexed shifter is the number of clicky clicks in it, that's all. They are not compatible among different numbers of speeds.
Aside from some serendipitous combinations, anyway.
3. A friction shifter can run anything.
Assuming it can sweep the entire cluster, yes. But the more sprockets you pile onto the rear hub, the more deft your friction shifting must be to reliably hit the gear you want.