I started with a X300 on my G17, then switched to the APL. Yes, the G17/APL will fit in holsters made for the X300, but you end up with a longer/larger holster than needed. I can say from first-hand experience my new JM Custom OWB holster is far more compact than any of the holsters I have seen made for the X300. John uses the minimum amount of material to make a functional holster, which results in a compact and more concealable rig (see photo below).
My APL has thus far remained attached to my G17, and unlike the youtube video of the APL flying off several different pistols, I am unable to pull if off my Glock by hand. Just as some cheap-and-easy insurance, I placed Scotch double-sided tape (the thin kind) on the rails of the Glock and then installed the APL. If it wasn't going anywhere before, it definitely isn't now!