Games that size are tough, because so much of your force is in your commander. At that size (about 500 points I think), nearly half of the imperial side is Vader, and yes, you need to pile upgrades on the rebels to make the points even.

Doesn't mean you can't have a fun game, but realize that the commanders are going to dominate. Either they go off and eat most of the other units, or even more often, they duel and the winner of the commander duel takes the match.

You could think about getting Leia and Veers for the price of about one trooper expansion. Although lower points, it might actually make games more tactical and less a lightsaber die rolloff.

I'd also recommend good cover and LOS blocking terrain. This tends to provide more tactical choices and options. Small battles on small tables with just barricades quickly become just a die rolling party.