Is it balanced to have, say, a Stormtrooper, Commissar, Sergeant and Weapon Specialist in the same party, or are they for different powerlevels? I have no mind for balance and the book isn't clear on it.

Well the sergeant and the Commisioner for example have very different jobs to do. The Sergeant is very good at making his troops fight more effectively, while the Commisar is a superb warrior with some methods to enforce discipline. But, there is a big range in powerlevels, a wise gm guides the procves to build a balanced party.

I thought the game was fairly balanced between the 2 myself, afterall that extra 300xp could probably buy (can't recall the costs) the Sergeant at least 1 Sweeping Order, making them have a fairly useful leadership power.

And I made a Commissar today to try out character creation, and that 300xp that the Commissar gets total doesn't buy you much, for me only an Agility advancement and Quick Draw.