Field Commander

This is quite an expensive unit but very useful as part of a strategy. It gives a 25% health bonus to all units in the battle he is participating in as well as increasing their sight range. It is not very good for direct combat (only the field commander and a group of troopers) but will give you an edge in any ground battle.

Fleet Commander

Like the field commander, it is quite expensive but also very useful. It gives a 25% health bonus to all ships in the battle he is participating in as well as increasing their sight range. He will be more protected than the field commander because he takes residence aboard the biggest capital ship in the fleet; unless of course you have no fleet in which case he will be onboard a shuttle.

Smugglers

You can gain access to smugglers on Nal Hutta, Tatooine, Corellia, Abregado Rae and Atzerri provided you build a cantina first. Smugglers can be used to steal credits from the enemy when you insert them in the steal spot on any enemy planets.

Bounty Hunters

You can gain access to bounty hunters on Nal Hutta, Tatooine, Abregado and Atzerri provided you built a Hutt Palace. They can be used to assassinate other bounty hunters (not Boba Fett though), smugglers (not Han or Chewie), field commanders (not Veers) and fleet commanders (not Piett or Ackbar). Not all that great, but quite useful nonetheless.