I can't see the Stomach of Steel description. It's the steady stomach feat?
I really like the Bard substitutions.
Gaseous form as a spell like abilty contrasts a bit with a total-non spellcaster class. And for a rougue is incredibly useful, and they will use this like costantly, thrust me, I've done exactly like this with my old rogue.

Nice addition. Are you going to make substitution levels for the other classes too?