I just bought IA with some of the Villainy/Ally packs.

Played through the campaign once on the Hero side (not my copy) and had a blast.

Now I will play the Imperial side against a friend (controlling 4 heroes).

But what about Twin Shadows?
Does everything integrate?
Or is the Twin Shadows a totally separate campaign?

Not sure anyone knows for certain how this is going to work, but the information that we have seen is that it is a 'mini-campaign.' People generally think that this will be done in the form of side-mission cards that will be tied together in some fashion, but which can be played in the main campaign if desired.

Someone else may know more.

It comes with new heroes, imperial class(es?), agenda cards, and side missions that can be used in the main campaign. Heavy Stormtroopers and Tusken Raiders most likely as well but we're not 100% sure. Or you can also string the new side missions into a separate mini campaign if you like.

I for one will wait for the new stuff before we begin another campaign...

Do we know how many missions are in the new "mini campaign"? I'm wondering if it's "mini" just because it doesn't branch with each mission outcome; if there are enough of them, one could presumably play those missions in place of the core "story" missions and still play core side missions in between them.

Do we know how many missions are in the new "mini campaign"?

The Twin Shadows box will have 6 missions.

They can be played as a mini campaign with side missions from the core box in between (probably totalling 8-9 missions).

Also they can be played as side missions in the core box campaign.