I have never had any success. Tried just one colour, 1 filament is loaded into the chameleon. It prints the first layer, but that's about it. I think it's cause maybe the filament switching gradually becomes weaker and can't drive the system.
I'm using a prusa mini btw with prusa slicer 2.3.3 cause the reversedrive doesn't like the temp files in 2.4.1 or 2.4.2.
When you say it gradually becomes weaker... can you describe what you're seeing? The motor should stay on and locked to provide tension. Also, have you tried any of the other extruders? For example, set it to T2 and print with it. (I guess I should answer your first question, too: no, it doesn't need to all be loaded.) Lastly, the reverse drive code has been updated for 2.4.x... have you downloaded the most recent version? Prusa did change their code to and renamed their temporary files with a .pp extension in them, but I updated it a few weeks ago to handle that case.