I think Magic Guard has some real merit here, if not a little too much.
Defensively, it essentially adds on a third status immunity (Poison/Toxic) which gives AT Smokomodo incredible survivability. It removes the Fire type's Stealth Rock weakness which is a major detriment of the typing in the current meta, allowing this mon to carry an item other than HDB.
Offensively, it enables a lot of physical offensive potential. It removes the recoil and crash damage from Flare Blitz and Supercell Slam, enabling this mon to throw out powerful stab moves with significant longevity. This also pairs well with the Life Orb, providing a free boost to these moves. I think this ability is extremely well suited for this typing but may potentially be too powerful, I'd like to see what other people think.
A few lower power abilities I could see are Strong Jaw / Tough Claws since both typings have Biting or Punching moves and also opens room for strong coverage.