As the topic states, is it possible?
They cound as medium armor. So RAW it should work. Or are I missing something?
No you can't. Yes you are. I'm not entirely sure where it is, but there is someplace that points out that 'armor that counts as lighter' only affects certain points, and not things like armored kilts which function because they add armored areas to areas not traditionally armored by actual medium armors, rather than just lighter heavy armors.
I'm not so sure. With mithral full plate I'd say you are right. For this you would need heavy armor prof even if it is treated as medium armor.
But elven chain, celestial armor and celestial plate armor is treated as lighter in ALL aspects.
Perhaps RAW, you may be correct. I'm 2000 miles away from my books, so I'll assume you've read the relevant passages. Balance-wise, the increased armor bonus is offset by the lowered max Dex. So I'd not contest you on either of those portions.
However, the intent behind armored kilts providing additional protection to medium armors and lower is the amount of physical protection provided by a 'full plate' armor suit vs a 'chain mail' armor suit or 'breastplate'. Because those armors tend to neglect that area, or provide less effective protection to it, an armored kilt provides an effective new piece of armor for someone wearing that suit, and not for someone wearing armor classified as 'heavy'.