Rear derailleurs don't care about how many gears you have in back. They move however far the cable is pulled. If your shifter pulls the derailleur the width of space to the next gear, you get a smooth shift. if not ....

If you have say, a Tiagra 4600 (10-speed, with the cable exiting the side of the upper shifter body, so it should work) and a nine-speed cluster you get a dead click. You start inthe lowest gear (largest cog) and drop down the cassette (up in gerars) as you accelerate. You hit the top (smallest) cog----but you try to shift again because you lost count. The shifter pulls but because you have limited it with the high limit screw, it goes nowhere.

Then the road tilts up a little and you hit the rear shifter once to drop down one gear ... and nothing happens, because it is the "ghost" gear, the tenth gear which isn't there. So you have to shift it twice. That's about as bad as it gets---nio mechanical harm done that I have ever heard.

(I have Claris shifters (8-speed) pulling an MTB derailleur across a 7-speed cluster----That has problems because of gear spacing differences between 7 and 8 .... but it still shifts pretty well, though it does need that double shift to get things moving at either end ... which can be annoying until one grows used to it. Having to constantly do "shifts-and-a-half" is much more of a pain, but old 7-speed RSX shifters are nearly impossible to find.)

Warning: My knowledge is limited and you would do well to get more opinions than mine and discard mine, like throwing out the lowest score in Olympic judging. Enjoy the performance, complain abut its flaws, reward the better performances.