Personally, I'm still confused by how Ash who was a non-com in ME1 & 2 is now suddenly a LtCmdr. What was the time span supposed to be between ME2 & 3? Six months? A year at the outside? That's a hell of a promotion. Actually, now that I think about it, doesn't Kaiden (for those who didn't nuke him of course) now outrank Shepard? I wonder how that will be addressed.
Regarding Ash, I've heard that...
And yeah, according to the Alliance rank system, Kaidan does officially outrank Shepard in ME3. But Shepard is probably still CO of the Normandy
, so at least while they're both on the ship, Kaidan would probably follow Shepard's lead.