DaveC wrote:
I've got an old 20 gal (?) compressor tank in which I want to install a
drain. I'm guessing it to be 1/4-inch (6.3 mm?) thick steel. Tank is rated to
125 psi.

Can I drill and tap this for 1/4 or 3/8-inch pipe? Or should i get a "plug"
welded to it that I can drill & tap?

Are there rules to follow for such stuff? Is there a better n.g. to ask this
question in?

A welded iron half-coupling is the correct way to go. Do it yourself if
you can. Your tank isn't worth what a shop would have to charge--about
$80 or their one hour minimum.
good luck,
Boat_dreams