I disagree with the above. I've played the demo an awful lot now, and have messed about with all kinds of tactics. As long as the rebels know what the enemy have, and build the appropriate units, they will win easily.
The smaller Imperials ships are awful to fighter squadrons. I've been able to take out 12 of them with eight squads of xwings and 8 squads of awings, with minimual loss. It gets tricky when the Imperials have a fair few of those small anti fighter ships. Those are a real pain in the ass, they take down fighters so easily.
I find ignoring the fighters and bombers and concentrating on the larger ships the best tactic. I tend to build a fair few corvettes to help protect the larger vessels. Use the long range missile ships. If you have 3-5 of those they take down Star Destroyers in no time. The larger ships should be a screen and take on the large Star Destroyers, using the missile ships to produce a barrage effect. The fighters should go in and take down the smaller ships, ignoring the fighters. While corvettes protect the larger ships from bombers.
First goal as Rebels is to take down the anti fighter ships though. If you don't you'll lose big time. Y wings aren't really needed, i find their armour to be pretty weak. But if you do use them, they need to go in fire and then fall back straight away.