It seems this would work best objective based. The Imps must take over the frieghter in a time limit, and the Wookies must defend for a certain time:
Wookies only have two classes, Warrior (stronger, slower) and Attacker (faster, weaker). Both have various types of melee attacks. The Wookies can also pick up melee weapons from around the ship (such as pieces of metal etc, to bash the IMps with).
Since the Imps are on a time limit, they have to attack, allowing the wookies to set ambushes etc. Imp kits can be picked up by Wookies, giving the Wooks weapons.
Some weapon restrictions would have to be in place, e/g grenades vs unarmed soldiers would be very unfair. Maybe stun grenades instead?
Finally, the Wookies could have an alternative way to win - they can capture a transport, fly to the hangar and destroy a tractor beam generator, allowing the freighter to escape.
--------------------------------------
If I should die, think only this of me:
That in some corner of a foreign field
There lies a plagiarist.