So yesterday, I was thinking, "We don't have any FFG invented ally. Why?" And then it hit me, "Duh, heroes fill that slot of FFG created Rebels!" But if we wanted some, couldn't we use red missions instead of green, but if the rebels win they get the skirmish version of the hero? Of course it wouldn't work for every hero, but it would for most. You'd have to add their cost in threat, and there would likely be more errata, but it could be fun!

Could work, but some of the Rebel heroes have abilities that are specific to Skirmish, like Saska, Fenn or Diala. Other than that I don't see why you couldn't.

I came up with some house rules for playing with a 5th rebel player using the skirmish deployment cards of the Heroes.

Allow a 5th rebel to play as an ally over and over again. If they get eliminated, let them re-deploy on the next round and give the Imperial equal threat.

There are still some issues with this, as some of the skirmish variants have little life and can get killed off easily in 1 activation. Perhaps give them double life and treat the first "death" as wounded by removing an ability. The skirmish variants become increasingly weak as the campaign goes on as well.

I also came up with the following house rules for the units that have made that are skirmish specific.

If you have Diala as an ally, give her two attacks to compensate for the lack of Shattering Blow or Make Shattering Blow cost 2 actions (I can't decide). Mak's Critical hit gets Blast 1 if it defeats a hostile figure. Fenn's Trooper Assault, he doesn't need it, he's tough enough. Murne's Field Report just allows her to become hidden. And Saska can do fine without Shady Contacts.

Hope that helps or gives you some framework for creating your own ideas.