Haven't seen any discussion about this, but who do you think they are?
They're not combine, because they're fleeing from the Strider.
They could be resistance fighters and were replaced by the guys in white coats later on, but they're supposed to be civilian (I think) and these guys wear military uniforms. They also have some kind of gasmask like the combine, but they can't be combine because they're running from them.
Could they be the good guy police of military force of the country/city-17?
Another idea that comes in my mind that combine soldiers are biomechanised versions of these guys and they are turned into mindless zombie soldiers.