I use them in my APQS. I tried it long ago with the disk out of the bobbin case and it didn't work. A couple of years ago the disk started to lose its "spring" and I had heard to add the genie with the disk. Works great!
As for size, I think originally they were only made in the L size and a LQS had a gammill(M bobbin) and used the L genie with the M bobbin for many years. That's where I first learned of them.