From gen 4 back yes. Unfortunately you can't use gen 4 mags in a gen 3 or 2 or 1 guns.

You can use the larger mags in the smaller guns but not the other way.

Yes and no, you can use a magazine that is longer then the grip such as a G17 magazine in a G19 but not the other way a G19 magazine will not work in a G17. All generations of magazines will work in all generations of pistols, as in a gen 3 magazine will work in a gen 4 pistol and vice a versa.

You can see a chart by going to the following:
http://eu.glock.com/english/magazines_compat.htm

Sean B said it like I believe it is.

Not entirely. 9mm Glock mags are interchangeable, so long as the mag is at least as long as the frame. The Glock 26 can take every 9mm mag, the 19 can take every mag except the 26, the 17 can take the 17 and 18 mags. And the 43 mag will only work in the 43.