Cargo ships could have been kamikazis (or still loaded with weapons) - besides, the rebels can't be picky when it comes to ships.

Speaking of which, the rebels need a LOT more ships in order to survive:

(1) potential Executor volley
(2) massive amounts of TIEs and SDs pummelling them
(3) DS2 raking super laser through multiple ships.

I would almost decrease the number of Imperial SDs (being that if this is still a trap, you can't exactly hide a few hundred mile long space ships) while increasing the number of rebel ships. I think someone else suggested showing 2 more rebel fleets jumping in from hyperspace as if to rendevous with the first fleet (where our heroes are) to show that the rebels are coming from different parts of the galaxy to converge at one point. I know they mention meeting at Sullust, but again, you can't keep a secret with hundreds of mile long space ships just floating around.

I would also add a lot more rebel fighters, of which a lot get blown up from more accurate TIE fighters -as in, show the Imperial force as an actual trained force that stands a chance of defeating the rebels. Always showing TIEs getting blown up by the Falcon or other rebels again tears down any credibility they have as an actual threat.