All glocks have 17 pound recoil springs so that weak handloads will cycle them. The Glock 20/21 frame is the same and the barrel will interchange. The slide of the 20 is about 1/3 of an ounce heavier than that of the 21. A heavier recoil spring will compensate.
The frame of the 29 and 30 is the same story, I don't know about the slides. Wikipedia had a chart comparing the weights of all components of every model at one time. Through spelunking the bowels of the internet I came across the info on the recoil springs but I don't remember where now.
I just checked on my Glock 30 and the magazine of the 20 WILL fit in it. (I have Glock 20 mags for foochure conversion)
http://www.youtube.com/watch?v=Wjhit...9v7MeFRpB-nYSg
Extractor/ejector info.
http://www.youtube.com/watch?v=0eMbPDQp6a4
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