You don´t _need_ barracks in a PDC at all. If you can prevent an enemy (assuming single race on the planet) from landing troops, you are fine (The crew will try to defend the PDC, but a couple hundred crewmen will not fare well vs. a battalion of power armored infantry).
And if you kill any enemy ship before they can start shooting at your planet (or intercept all inbound missiles) you also don´t need their bunker function.
Of course, those are some pretty big IFs
As for your second question, yes, you can restock the ammo of your PDCs from the planteary stockpile, but IMO this is _very_ gamey.