Oddly enough, Pixies can shoot heavy weapons just fine. They do have to get them customized, but yeah, they can use Panther XXLs just fine due to the well-thought out firearms rules. There's nothing wrong with melee pixie though, mind you. Other things they can do well are pure magician of either a Charisma or Intuition tradition, physical-combat oriented Mystic Adept, face mystic adept, and adept rigger. They actually aren't very good hackers because hackers don't care much about stats but do care about being able to use cyberware; Intuition is important for riggers, though, and Pixies are one of very few ways to get an Intuition bonus.
There's a big difference between "that's a pixie, it doesn't have civil rights" and "that's a pixie wearing a super illegal set of armor." If your GM isn't going to enforce that difference, then yes, you definitely want MilSpec.