1. a number on the push fit is usually the od pipe it'll take. for that size ram i'd guess 1/4 aswell, if you want to be sure of the port size just unscrew the push fit and check.

2. i'd use either 10 or 12mm for the pipes going from buffer to valve to ram, the tube from the regulator to buffer tank you can use smaller like 6mm.

3. with a screw top paintball bottle refill the same as you would with any other screw top bottle. assuming that's the reg i sold you ages ago you shouldn't need an adapter for filling up your paintball bottle