Gen 4 magazines can be used in any generation of Glock pistol, however you must use Gen 4 magazines in a Gen 4 Glock if you want to switch the gun from a right-handed magazine release to a left-handed release.
Use only Glock factory magazines. There are a number of knockoff, aftermarket magazines available on the market. Currently Glock magazines are inexpensive and plentiful. There isn’t any reason not to use the real thing.
You can use the high-capacity 33-round 9mm and 31-round .40 S&W magazines in full-size (Glock 17 & 22) and compact-size (Glock 19 & 23), however these magazines may or may not be reliable in the sub-compact frame (Glock 26 & 27) pistols.
You can use any caliber appropriate magazine in any smaller version Glock than what the magazine was originally intended for (with the exception of the new single-stack Glock 43). Example: In a subcompact Glock 26 you can shoot larger Glock 19 or 17 magazines. A compact Glock 19 can use magazines from a full-size Glock 17.
If you add a magazine extension intended to add two rounds to 9mm magazines or one round to a .40 S&W magazines, you will not have to change springs in subcompact magazines (Glock 26 & 27), you will need to use a spring intended for 17-rounds with compact size magazines (Glock 19 & 23), and if you want to add the extension to full-size magazines (Glock 17 & 23) you can order directly from Glock a spring that is intended to push 19-rounds. Only use genuine Glock parts, including the extension.
If you don’t have the patience for a detailed explanation, then you will still be well served by the information above. If you want to know more, keep reading.
I just bought seven of these. I love the M-16/M-4 versions and have patiently waited for an offering for the Kalashnikov. They feature a removable floorplate, constant curve geometry, and high-reliability/low-friction follower for the performance you expect from a PMAG. Upon initial inspection they appear to be quality gear, however I am a little suspect that the magazine locking tab is not steel. This is always a weak point on polymer Kalashnikov magazines. According to the internet rumor mill the next generation of the magazine will have steel reinforced feed lips and tab.