ER32 is very large physically. This was the reason for the design above. it will gain you 2 to 3 inches of clearance over the standard chucks. It also decreases leverage from the tool significantly, reducing chatter and strain on the spindle.
ISO20 is very small at 22.23mm diametre, and nowhere remotely near the strength and rigidity of R8. BT30 is the R8 equivalent in terms of size. In terms of end mills in a collet chuck, 3/8" is about the max you'll likely use on this spindle, but LENGTH is a far more critical dimension.