Yes, so long as they comply with the national regulating body's rules on rugby boots. I don't know where you're from, but my national body is the IRFU. This is not a link to their site, but...
Its because they have studs that can aid grip in the grass and muddy conditions that the game is played in
Yes but the referee will normally request they are changed as they are not the right foot wear for the game
he wears nike mercurial vapor ivs
Manchester United midfielder Christiano Ronaldo wears Nike Vapour IV's.