You need 7 (seven) split rivets. 6 for the sides, 1 for the crotch. 8-9 mm
The holster and belt is rivetet on with pop rivets - the same as the helmet. People often use Chicago screws; the look the same
I'm not sure about the knee, but the look a tid smaller than the split rivets. I'd guess 6mm, standard rivet
Edit: The plastic belt has 3 rivet covers, so I'd say 3 fer the belt