If you do go for nails,,,,, one and a half inch galvanised nails (broad headed) will give you a good hold..... Most plasterboard screws are put in using a screw gun.
Just to add to Nose's comments, 42mm or 50mm screws will be fine with 1/2 inch boards. Using screws will prevent the roof tiles/slates 'popping'. I must admit I've not used clouts for years.
For 12.5mm plasterboard, 40mm 'ish plasterboard screws would be ok. Space them at 7 or 8" apart, especially on a ceiling, and make sure they're in just slightly below the surface of the plasterboard if you're going to tape the ceiling. There's nothing worse than the screw heads not being in far enough.
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