Aye, Mage Armor doesn't stack with other Armor bonuses, though it does stack with Shield bonuses in 3.5, so you could have a large/heavy mithral shield and use it with Mage Armor active. Mage Armor does have the benefit of no Arcane Spell Failure, and functioning against incorporeal touch attacks, unlike physical armors.... You could instead keep your mithral chain shirt and take the Shield spell in place of Mage Armor, since 3.5 changed it to simply a shorter-lasting copy of Mage Armor that grants a +4 Shield bonus to AC. Some things I noticed when updating Seruleus to 3.5 recently, since Seruleus knows the Shield spell.