Would they stack?
When you add an armored kilt to a suit of light armor, the set counts as medium armor. Likewise, a kilt and medium armor counts as heavy armor.
Mage Armor adds a +4 Armor bonus, but does not actually count as Light or Medium armor.
How do these rules interact (if at all)?
(The highlighting is done by me and not part of the RAW)
[...]An invisible but tangible field of force surrounds the subject of a mage armor spell, providing a +4 armor bonus to AC.
If you are wearing some form of armor, the higher armor bonus counts (either armor + kilt or mage armor) since they are both armor bonusses and do not stack. You cannot apply a Kilt to Mage Armor since Mage Armor is neither light nor medium armor.