I figured maybe the grub screw ones would have a metal collet or something while the push on ones would have nylon rings or something softer to grip the splines... not sure though. I’ve sent an email to Fuzzdog.
In my experience, push on are splined to grip the splines and grub screw ones are made to accept the shafts that have one flat side. I'm sure they're not the only options though!
I can't see why not. 6.3mm is 6.3mm. Go for it!
*I'm not an expert or nuffink...