Will the 7 round magazine designed for the S&W CS9 fit and function in a S&W 908/3913 ? I can't get one to seat and hold in a 908 so I'm assuming these are too short, and only the 8 round mags work.
True. Only 8-round mags (for the 3913/14 guns) work and fit flush in your pistol.
If you get a 3906 mag, it will work but stick out the bottom of the grip.