It does not stack with "DR from other sources", but stack with "DR from abilities" (such as Stalwart Defender Ability).
What do we have when stalwart defender with "Stalwart" feat uses combat expertise wearing adamantine full-plate?
Depends, what level is the Stalwart Defender? At level 10 you're looking at DR 15/- with Improved Stalwart you're looking at DR 20/- Assuming that you're in Defensive Stance and took Improved Damage reduction. Otherwise DR 18/- for that round. So long as you aren't denied your dexterity bonus to AC.
OK, so armor bonus to DR stack with theese.
Thanks!
Adamantine DR wouldn't stack with Stalwart. Since Adamantine Armor isn't an ability, it wouldn't stack, so you'd take the better of the 2.