From what I know the combine arn't a 'thing' really. They are influence.
I think that the citidel (sp?) is a invation ship that is droped from the sky and it just eats everything around it. The combine use conquered species as wepons, like the dropship and strider. Those are living things that have been made into wepons. Same thing with the metro cops, those are humans who are under the combine influence. Those strange walking things in the citidel are vorgons.
the combine "assimilated" all their tecnology. I see the force on earth as a slave army that is controlled by the puppet masters (look at the screen when you reach the top of the citidel).
yeh dude you are right, but what makes me wonder alot is that, at the end, when breen takes your grav gun, he runs away right, if you follow him, you will see him talking to a big worm-like creature, then he flees away down the elevator, who the hell is the worm??? Maby IT gives breen the monsters to turn them into robots?? And about the G-Man, everyone seems to know him except Freeman, WHY!?!? He is one freakin Guy, And at the END END END of the game, where the hell does the G-Man go? He throws himself from a white window or something?? And where did freeman go? Followed him down or what?? Very ironic game i say!!!