Well..
Mercenary is fairly traditional ranges dps with casting time. They have access to healing.
Powertech is really melee with a little long range instant cast on top. He has tanking stance.
Marauder is just melee dps with little more spec option within melee dps.
Juggernaut you mentioned briefly. Melee dps with tanking stance.
I hope you know you wont be switching between the advanced classes on your character.