Dont forget that 4 hokage not only sealed kyubi in naruto because he was dangerous but for power, the jin are weapons and iven jiraya sayed that the seal is not weakening the kyubi chakra but allows to use it. But then you could be right but then why he used child ?? Why he dont use the shinobi with preety much chakra but baby ?? I think that because in years are past they chakra is mixed and its easier to use it or in fact be controlled by it so why he chose the child if he only whant to seal kyubi ??