That's fine. Your isolating the PICs supply from that used for other significant switching loads, which is the whole idea. What that 'other' supply becomes, is irrelevant.

I really can't comment on the size of your Capacitors without knowing what your total worst-case load will be and what is actually going to be used as the Regulator. Build it and see...

If it's an automotive application, you should really be using 12v Relays. All you are accomplishing (other than using up your stock of 5v components) is heating up the planet! When the ice caps melt, it's now public knowledge that it'll be YOUR fault.