As I understand, a G17 mag will fit in the G19...but, it would hang down, of course. Is there a sleeve to fill in that space...to make the extra length of the longer mag become part of the pistol grip?
I occasionally use G17 mags in smaller guns (G19 and G26) without a sleeve - I used a 26 in a competition with G17 mags. I personally haven't found that I need a sleeve. But the guy I know who use them use the X-Grip.