that about sums it up.
Yes, they only count as medium for proficiency.
It counts as light for every purpose except armor proficiency I believe.
So, Mithral Breastplate, add the Brawling enchantment, then add an Armored Kilt.
What happens?
actually very good question, i would like to know as well cause who knows when it may come up?
well Bt RAW you would be fine. It says you can only PLACE the brawling enchantment on light armor if that armor becomes heavy or meduim the brawling still functions.
I'm going to go against the flow and say "no" to the +1 brawling mithral breastplate. My reason is because what it actually says on pgs. 154-155 of the CRB is that mithral armors are treated as one category lighter than normal for purposes of movement and other limitations. Then they say that the decrease doesn't apply to proficiency, which tells me that it still is what it is, just lighter and easier to wear than normal. Clearly, the way that most people remember it is as mithral being one category lighter for everything but proficiency; but that's not actually what it says.
If I allowed armored kilts at all -- which I wouldn't, and PFS doesn't -- then a +1 brawling armored kilt would work just fine with celestial armor, for the same reason. I would just treat the combination as one category heavier than normal for purposes of movement and other limitations.
Yes, that's half of my point: it's not so clear-cut as people are making it sound. The other half is that if proficiency doesn't count for the decrease, then why should magic special abilities? Even though a mithral breastplate is lighter and easier to use, a bard still can't wear it without penalties for non-proficiency, because it's still medium armor. So RAI, how would you justify putting brawling on it?