There's more weight in a polymer-pistol (Glocks) slide than there is in a all-metal 10mm 1911 so that tames down the 10mm more than in a 10mm 1911. It's all about physics.
Plus, the slide on a G20 is a little heavier than the G21 slide , an with the 10mm that is more powerful than a .45 acp bumping up to a 20 lb. recoils spring (G21 = 17 lb.) would be best.
Also, conversion barrels between the G21 and G20 have differences in the chamber hood width. The 10mm barrel hoods are about 0.427" wide at the back. The .45 ACP is about 0.480", so a 10mm conversion barrel has the same width as the .45, and visa versa if going from 10mm to .45.
A G20 factory barrel in a G21 slide looks like this.
View attachment 56061
A G21 10mm conversion barrel looks the same as this .45 acp factory barrel in a G21.
View attachment 56062
Also, if you want to shoot .40 S&W get another conversion barrel that fits the hood width of either the G20 or G21.
Also, G40 barrels will fit in a G20 but stick out, and the same applies to the G41/G21.
BARREL............FITS PISTOL(s)
G17 preGen5.....G17 preGen5
G17 Gen5...........G17 Gen5 + G19, G26, G45, G47
G17L..................G17L + G17 preGen5, G34 preGen5
G18....................G18
G19....................G19 + G26, G45
G20....................G20
G21....................G21
G22....................G22 + G31
G23....................G23 + G27, G32, G33
G24....................G24 + G22, G31, G35
G25....................G25 + G28
G26....................G26
G27....................G27 + G33
G28....................G28
G29....................G29
G30....................G30
G30S..................G30S + G36
G31....................G31 + G22
G32....................G32 + G23, G27, G33
G33....................G33 + G27
G34 preGen5.....G34 preGen5 + G17 preGen5
G34 Gen5.....G34 Gen5 + G17 Gen5, G19, G26, G45, G47
G35...................G35 + G22, G31
G36...................G36 + G30S
G37...................G37
G38...................G38 + G39
G39...................G39
G40...................G40 + G20
G41....................G41
G42....................G42
G43....................G43
G45....................G45 + G19, G26
G46....................G46
G47....................G47 + G17 Gen5, G19, G26, G45
G48....................G48 + G43