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you can't have 2 sets of armour so no and unless otherwise stated in the armoury then no to your second question as well
"take my love, take my land, take me to where i cannot stand, i don't care i'm still free you can't take the sky from meee" ;- firefly
"i'm gonna grant you your greatest wish i'm gonna show you a world without sin" ;- serentiy
"I am a leaf on the wind, watch how i soar" ;- serenity
Yeah, armour is a one set only deal, I believe it probably says that in the main rulebook in the magic items section, under magic armour. And the bosses hat doesn't give armour, only those helmets that specify this (Not in the orc army at all) give him the extra boost.
good equipment for a warboss is ironskin shield, fumpas club of fumpin, heavy armour, and stuff him on a pig. he's got a good ward save, armour save, rips through armour and wounds almost anything on a one.
Mmmmm...Snotlings... good for breakfast, lunch, and dinner!!!