I'm likely way off but I think Sarge is some sort of energy life form and can take different forms, him and Izel are probably the same species.
Also I don't believe Izel was on the spaceship but had somehow hitched a ride with Davies, I also think she has switched from him to May.
That seemed so obvious that I began wondering if they were trying to create a false lead.
I'm sticking with him either actually being her -- i.e. she's a shape shifter or is able to convince people to see what she wants them to see rather than what their eyes see -- or he's being possessed by her in some way (which goes with the "bat-control" we've already seen).
Yeah, mostly liked the episode but for a couple of things. The switcheroo was very obvious, why did Jocko commit suicide (effectively) rather than just, er, put the bomb through then close the portal, and the "Are all women on your planet this powerful?" line. Or, to be more exact, the "They are" follow-up, which is just infantile [imagine an alien male asking that after seeing Superman].
Basics were sound, though, and it was quite fun watching Mack and 'Coulson' knock seven bells out of each other.