Iron skin + actual armor

[Xyllen]

if a wizard somehow wears say a breastplate do the two stack?

Iron Skin (Su): You gain a +2 armor bonus to your Armor Class for 10 minutes; this bonus increases by +1 for every 5 caster levels (maximum +6 at 20th level). You can use this ability a number of times per day equal to 3 + your Intelligence bonus.

[Brain in a Jar]

No. Armor bonuses don't stack. You would take the higher bonus.