So i have character with proficiency in Medium Armor.
If i buy Mithral Breastplate and attach Armored Kilt to it, will i get medium armor that grants +7 AC? If yes, does kilt have to be mithral too?
This question again...
Mithril Breastplate is considered Light Armor for everything outside proficiency.
Armored Kilt can be attached to any armor (besides Heavy Armor), and raise its armor category up to the next level. So here you're left with an armor that, while is considered Medium Armor for everything, Mithril property does not deduct its proficiency requirement to Medium again; thusly, you need Heavy Armor Proficiency to use it properly.
Also, the Armored Kilt doesn't have to be Mithril, though even if it was, it wouldn't apply to itself or the armor you have equipped since you can only utilize one special material per slot at any given time, and items made half and half (say, Mithril Breastplate and Adamantine Armored Kilt) only uses the most prevalent material (since Adamantine Armored Kilt is attached to Mithril Breasplate, the Kilt is inferior, and thusly not used).
**Edit** May I recommend you attach this to Celestial Armor to get the effect you're looking for.
Thanks, that is what i thought. Despite that i believe wording in mithral is made with other purpose, they still prohibiting use of such combination :(