Here's something I did when I was modding this scenario... use it if you think it's interesting.
I gave each of the human groups a special 'sniping' unit. They had high bombard, a range of 2 or 3, and had a pretty good rate of fire, which, if you're lucky, will take out a zombie (to represent a head shot that knocks off then zombie's head). To counterbalance this, they had lower attack and defense and were a bit costly, so that they don't become too powerful.
As for names, I think I called the Survivor unit a 'Hunter' (guy who enjoys hunting recreationally on the mainland, but now is hunting zombies to survive), the Company unit a 'Mercenary' (pretty self-explanatory), and the Authority unit a 'Sniper' (really self-explanatory).