That’s more less what I was thinking. My vote would be on pirates - actual rapists, plunderers and murderers. Or, some kind of secret cult. A bunch of religious fanatics - perfect fit for a world in desperate situation, and it would be pretty easy to make them visually unique.
Good point about the sides looking very similar, I wonder if there are any gameplay differences between security and resistance. If there are none, then I don’t see the point in adding more. It can already be hard to distinguish security from resistance, if both can be bare-chested, covered with tatoos, wearing riot armour (more worn out on resistance, but still similar). If you just want a bunch of new models, women could be nice. They could be restricted to light/medium body build, to make sure men retain their enviable manliness.