Truthfully, how many ex-Iraqi army can there be? Especially the ones that still want to fight us? Its now known that alot of the ex-Iraqi Army now make up the new IDF and Police Force. Really most of the Iraqis themselves are fed up with the violence and thier country being destroyed, they just want a safe place to call there own.
The ones that are fighting us is all of these foreign fighters that are coming from ALL of the surrounding countries, not just one or two because there are radical militants all throughout that region that are just itching to pick a fight, so if they dont find a cause in their own country they go to Iraq and or Afghanistan to 'prove' themselves, its an ancient and time honored tradition. This tradition is known as Jihad.
Now about these choppers. Its obvious that a big chunk of the helos that were successfully brought down were done so by MANPADS, with a few exceptions, and Heavy Machine Guns like the 12.7 and 14mm "AA" guns. These couldn't have just been found out in the desert somewhere by the insurgents, they come from outside suppliers such as Iran who has already been known to fund and supply and train these insurgents to use their sophisticated weaponry such as their MANPADS and Heavy Machine Guns, and their special remote detonated Shaped Charge HEAT mines and IEDs that the insurgents love so much.