So in a recent discussion it was brought to my attention that the rebel players can only bring in 1 ally to a mission during the campaign. I guess I must have missed that tidbit. I thought the Rebels can bring in as many allies that they have earned over the course of the campaign as long as they are willing to give the Imperial player all that threat.

Anyway, I was thinking of letting my Rebels do this as a house rule, because to us more iconic characters on the board = more fun, but would doing this severely imbalance the game?

I'm not sure it would imbalance the game, but what it would do is flood the board with figures. The two figures that seem like they should be together are Han and Chewie. That's 27 points of threat right off the bat. Or in other words 1 regular stormie squad 1 elite stormie squad, and 1 elite royal guards. All capable of spawning at the first spawn point. I would say it's hardly worth it.

The only concern I would have would be actually being unable to spend the threat if your open groups are all lower cost troops. If unspent, threat just sits in savings until used, right? To a maximum of 20?

It doesn't work - as there's not enough threat (capped at 20). We just played a mission where the rebels brought Han, and the mission started with Add 2x threat level (5) - so right off the bat I was maxed at 20, and I still managed to bring in my entire hand of figures (playing Imperial Might with Endless Legions, so a ton of Troopers/heavy Troopers and an EWeb). If the rebels brought in additional figures, they'd be overpowering the imperials, so you'd have to match it and bring in sufficiently more imperials and unique imperials to balance it, and at that point you clog up the map with figures. As it is, we spent 3 hours finishing that mission, and that was only with Han... (Canyon Run, so it included R2 and C3PO by default)

It doesn't work - as there's not enough threat (capped at 20). We just played a mission where the rebels brought Han, and the mission started with Add 2x threat level (5) - so right off the bat I was maxed at 20, and I still managed to bring in my entire hand of figures (playing Imperial Might with Endless Legions, so a ton of Troopers/heavy Troopers and an EWeb). If the rebels brought in additional figures, they'd be overpowering the imperials, so you'd have to match it and bring in sufficiently more imperials and unique imperials to balance it, and at that point you clog up the map with figures. As it is, we spent 3 hours finishing that mission, and that was only with Han... (Canyon Run, so it included R2 and C3PO by default)

Actually, the timing for when you gain threat from the special setup/rebel allies happens in two distinct phases, once during Step 3: Deployment and Setup, and again later during Step 7: Deploy Rebel Figures. Since you know if the rebels have an ally, and you also know the special setup rules, you should be able to deploy during Step 3 to prevent hitting 20 and are able to mitigate the issue of being "capped" at threat.

That said, if you wanted to house rule something like this, there would need to be a couple of concessions for the Imperial player. My suggestions would be:

1) Each Ally deployment would need to be done one at a time, to allow for a seperate Imperial threat gain and optional deployment for each Ally.

2) Each Ally brought beyond the first should allow the Imperial player to add another deployment card to his/her open group(s). This open group would be chosen after the rebel players declare the intention of bringing the additional ally, but before the ally is chosen or deployed.