I would not use any drywall under 1/2", 3/8" will tend to sag, 5/8" is usually what is used for comercial work. 1/2" will work for this, do not run the rock with the joists but across for better strength. On a ceiling I would suggest useing 1 1/4" drywall screws about 7" apart, making sure to sink the heads slightly below the surface of the rock. Don't run them in too deep. Make sure the ends of the rock(butt joints) hit center of the joist. Be sure to stager your drywall, if you start one side of the room with a full piece, then on the next run you will need to start with a half piece, and so on.