The reason for wanting a copper or brass mallet is that you can let
the weight of the mallet do the driving, rather than having to do it
with muscle, like you do with a lighter rawhide mallet.
Both will work, but the copper/brass one will work easier, so long
as you can hit accurately with it. I got my copper hammer (about 2
pounds) at Harbor Freight when they were having a sale. Think I paid
maybe $10 for it, a few years back. Keep an eye out, and you can
probably find one reasonably cheaply. (eventually)
The reason for using either copper/brass or rawhide mallets is that
way the striker is softer than the punch, and the end of the punch
doesn’t get deformed. Deformed punches have the magical ability to
leap across the room to screw up your bottom cutting plate when
you’re not looking. Best not to let them deform to begin with.
Do the cutting on a stump, not on an anvil. The first time you do it
with a copper/brass mallet, you’ll be surprised at how hard it hits.
Makes it really easy to pound the punch through the bottom plate,
and smash it into whatever the plate was sitting on. Best that be
something soft (ish). After the first few shots, you’ll get more of
a feel for how hard is ‘enough’ with your hammer.
You can also drive the disk cutter with a hydraulic press, if you
have one of those. (or a kick press, or even sideways in a big vise
(with copper jaw shields) if you get really desperate.)
All the cutting edge cares about is how many PSI of pressure it’s
seeing. Doesn’t much care how it got there, or in which direction.
Or even how fast, really.
For 20 gauge, I’d almost leave it hard. Cleaner cut that way, but I
don’t think it’ll really make much difference one way or the other.