Reserve ships deployed as part of a CPA will become threat that way, but the normal reserve defensive spawns are never supposed to go threat; they're supposed to stay on their planet and fight until the deployment is over, and then get reabsorbed into the reserve.
Where do AIs get the ships for CPAs? Only from Barracks, Reserves Special Forces and threat?
I don't recall off the top of my head if SF is eligible for CPA duty, and threat is not considered in spawning CPAs (since all a CPA is, is releasing guards into threat).
Guard ships are the traditional source of ships, and they're just the ones sitting around a guard post or whatever. But that source is probably eclipsed by the barracks and reserves now.
How much would destroying every Barracks in the galaxy, both Reserves and Special Forces affect CPAs sizes?
If there weren't enough guard ships, the CPA would come up short and just free what it could.
Also how fast do the Reserves grow? If I destroy all Reserve ships how long would it take before they're maxed again?
I don't have the numbers in front of me but if you have advanced logging on it will log the additions to the reserve in the special forces log file.