So here's how I currently understand things, let me know if any of this is wrong, because I'm trying to (over)simplify the veritable profulgence of options here.

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.

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.

3. A friction shifter can run anything.

I'm especially curious about number 3 because I'm thinking of using an old school friction shifter to control a modern 11 speed RD.

Cheers