1. You could certainly use that with a PACA or other tan soft armour carrier underneath, Ive seen a huge variety of chest rigs over soft armor in pics of PJs.
2. Well, ya. I know most people prefer other pistols like the sig or the usp, but the only issued pistol to the PJ's is the M9, and a lot of them dont even carry one.
3. hmmm check this for a good M4 PJ guide:
And I think 4 is somewhat answered by the above.