JNB, this is topic for Rebel's mission.
I've red that they added some starships in Imperial's mission, but not in Rebel's.
8 X-wings (IV lvl)
10 Y-wing (V lvl)
Adding 2 of each would solve the problem.
BTW, this thread has almost 2 months. Mayby i'm wrong, but I believe it needes 3 minutes to change 2 numbers in in-game's code to solve the problem. What's more, several days before they did it for Imperials, but Rebels stayed untouched.