Is anybody running the campaign with all 6 players? Does the missions become too easy with everyone playing?

Well, the rules don't actually allow you to play 6 heros. 4 heros max unless you make some very major house rules.

If you play it without any major house rules and follow the logic that all heroes have one activation, it will be very one sided to a point where you might as well just read the fluff for rebel victories and skip the actual gameplay. It will seriously be that broken without any house rules.

What would make it balanced? I have no idea. The problem with balancing is that you will not know if it is balanced or not, until you playtest it several times which is not fun.

However. If you are not looking for a balanced gamr and just want to have fun and recognize the issues then by all means try it. The main thing is to have fun! And you are the only one who can judge whether or not you are having fun.

We tried this a few weeks ago, and it was fun. We played Diala's story mission and lost because too many heroes got wounded. I was the Imperial Grand Admiral. All we did was increase the Threat level from 2 to 5. It seemed pretty balanced at first (maybe a bit too easy for the Rebels), until Vader appeared on the board...the Rebels managed to get 8 damage on him, but Diala was running too low on tokens and Vader was victorious.

That is perhaps a bit of a different scenario, since Vader is so overwhelming to low-level and poorly-equipped heroes. However, until Vader entered the fray I did think that the Imperials were short on firepower against 6 heroes, so in most missions it may be worthwhile to increase the threat even more than I did. But I do think that increasing the threat is probably the way to go if you want to use more than 4 heroes. It's just a matter of trial-and-error and feeling it out.

At 6 heroes, limit the rest action to once per activation (basically, get rid of the double-rest activation). This, with +2 threat per round I thought at an Empire player should be fair.

Of course, my Rebel players absolutely hate me, and think that they need to bring 6 heroes just to have a chance at beating me....heheheheehe.