The Glock 21SF does not require a different mag, it uses a 13 round standard Glock mag.
I once shot my Glock 21 side by side with a Kimber 1911 (a guy at the range traded with me for a few shots).
The Glock was SOOOOOOO SOFT, that Kimber with its hard steel frame and sharp cut checkered grips hurt my hand like crazy.
Glock 45's are SOFT shooters, period. You need to shoot one, we can hook up some time to do this.