Do you need to "unlock" the heroes besides the first four by doing missions and stuff or are they all available at start? Would it break the game to make them available if so? There are a bunch of players in my local group that really want to play other characters :)
It's pretty widely agreed that the Assault Team variant (as written in the core book, at least) is garbage. Nobody will play characters who get unlocked mid-campaign without catchup XP. So, yeah. It's not official, but just pick whichever 4 you want to play the campaign with. It won't break anything.
We started the campaign with Hawkwood, Kusanagi, Valeryia and Le Gong. The campaign is meant to be played with your starting characters. You will encounter other characters which can become allies, and join you on missions, but don't have the specialties, equipment and such available,. just the standard ally card. And yes, take a hacker.
Along the same lines, would it be feasible to have a group of more than 4 playing, but swapping out their characters depending on who shows up to play on a given day?
There are no rules, play as you like, CB doesn't care. From my experience, you'll have to figure out a balanced way for new players to acquire intel packs, skills, equipment, etc. If they all get the same levels and mission rewards you might run out of equipment to buy, and upgrade Defiance too fast. Also, if there would be some irregular party you might end up playing as a hacker all the time since its needed in most missions.