Getting ready to start our first campaign and I can't find anything that explicitly says whether or not you get to choose the heroic reward or if it's assigned. Since I don't own the game I'm not sure if there are multiple things to choose from. Thanks in advance.

There are only 3 "Heroic" reward cards, because you only use those when playing with exactly 3 heroes, and they're all identical. Similarly there are exactly 2 "Legendary" rewards which are even bigger bonuses that you use when playing with only 2 heroes.

That said, no matter how many humans will be on the rebel side (even if it's only one -- I'm playing a 1v1 campaign which has been really interesting) I strongly recommend bringing 4 heroes anyway, instead of using the Heroic or Legendary reward cards. It may be more things for fewer people to keep track of, but the missions are really designed for 4 heroes; some of them will be too easy with fewer rebels with huge health pools, while others will be too hard with not enough rebels to split up and go for different objectives.

So far based on 3 missions, 1 with 3 heroes and 2 one with 4 heroes I also recommend always using 4 heroes. While the Heroic and Legendary cards help with balancing, it is much better to have 4 heroes.

Thanks,
Duncan