Here's the story on the Glock ProMags:
The first ProMag Glock mags were unlined plastic, with witness holes, and available for G17s and G19s. These are junk, and only suitable for range toys.
Later, ProMag made some metal-lined Glock mags, with no witness holes, also in G17 and G19 versions. The G17 versions work great. The G19 versions don't work as well, because for some reason, ProMag changed the angle of the feedlips, causing the rounds to nose-down too much. They work acceptably with FMJ ammo, but not with JHPs.
You can use the G17 mags in G19s and G26s, and again, they work great, though they'll extend from the magwell.
-Troy