Kakero - No, you can use any weapon type available - the Monks didn't put a little cache of non-magical weapons in all those Chambers for nothing.
Malstein - No, the Brass Monks, like all the other Monk-types in the other Chambers, have no resistance at all to physical (slashing, piercing, bludgeoning or missile) damage. So attacking them with a Battle Axe (slashing damage) would have the exact same effects as attacking with a Warhammer (bludgeoning damage). Beats me why you'd get such different results with characters as similar as those, apart from the luck-factor...