Regarding under-armour stuff...
For the most part, better-armed and equipped soldiers would wear
at least a chainmail shirt under their coat of plates, both to
cushion the blows as well as to provide rudimentary protection to the
arms and armpits, thereby immediately "solving" the problem of the
rivets shredding clothes. However, due to the weight and
inconvenience, this practice (brig over mail) is very rare in the
SCA, since we're not really too concerned about an arrow punching
through a weak joint between the plates or a heavy blow popping
rivets in combat ;) To be honest, I don't fight too too often heavy,
but I have a Wisby Type 1 coat of plates and never had too much
trouble with the rivets tearing clothes, I just duct taped over the
rivets on the inside and it was done.
To solve the problem (and also for the less well-equipped men at
arms who would forgo mail hauberks entirely), you want to get
yourself either an arming doublet or some kind of gambeson. In
period this stared off first to go under mail to help cushion the
blows (sometimes worn by itself by footsoldiers), and later evolved
into an elaborate garment involving lacing points, strategic padding,
and patches of mail over various places like the armpits and groin in
order to reduce the weight of having to wear a full mail shirt.
Examples can be seen here:
If you want to make your own, you can check here, as they have
directions as to how to make one and period illustrations:
Hope that helps!