I don't think all officers start out as Ensigns, either. Does anyone know for sure? I mean, didn't Valeris and Saavik go to a type of Command Graduate School, and come out as Lieutenants? And I'm sure that graduates of Starfleet Medical (Bashir) were not Ensigns upon graduation. There must be a similar graduate-type school for Engineering....
As said above, Military Doctors are typically Lieutenants (I learned that from ER

...though I suspect that it wouldn't be unlikely for an officer trained to take the command route would have specific command training, or an extended course post Academy (explainng Saviks uniform colours) which would allow them to skip a rank as they'd be more qualified than an engineeing graduate.
It would be similar to the idea that if you've trained someone to be captain of a ship, then didn't offer his own own command you'd have wasted a lot of money in that training...
...I'm getting that idea from Down Periscope where Kelsey Grammer played Lt. Commander Dodge, the navy had invested a lot of money training him and not giving him hos own command would be a waste of funding... another TV/Movie reference there
