Well they will be legal on a soccer pitch. But this question really turn a couple of different issues -- the type of rugby boot you bought and the level of play your brother is at. There can be a significant difference between soccer and rugby boots. Soccer boots are intended (primarily) for speed, having just enough grip to allow the player to accelerate or change directions on turf. Rugby boots -- particularly those for the forward positions -- are intended primarily for the traction the players need for scrumming and rucking. The studs are larger -- huge by soccer standards -- albeit more rounded because you are also stepping on other players more often. The traction is great, but they do slow you down some. And they will look very odd on a soccer pitch. If it is casual pick-up soccer he'll be OK, particularly if the boots you bought are intended for a back, which means only six studs.. But if the league is competitive and your brother is going to have to fight for his spot, then you are going to want to replace them.
Make shure there not metal cleats if there plastic then they should be ok u should retern them as most refs won't allow them I play rugby for my high school and we have to wear soccer cleats because rugby ones are meatal and could hurt us
