Re: US Army CID Help
CIDC uses military and civilian investigators. The military personnel are enlisted, WO, and officers. The enlisted guys have to be at least an E-4 to apply. The civilian guys are 1811 series GS employees, and yes they can arrest civilians.
As far as organization goes, there are CID detachments on most major Army installations, and forward deployed to places like Kuwait, Iraq, Afghanistan, Qatar, etc.
The size of specific sub-units isn't going to be available to the general public. If all you want this for is game play, I wouldn't sweat it, and just go with whatever you feel fits the bill for your game.