Are you allowed to replace a person that has been red carded in a rugby union match?

technically no. Once a yellow has been shown the player is off for the 10 minutes. However, where that play is classed as a speciality position (only one of the front row) a reserve may be fielded but another play must leave to make room for them. In the case of a front row player having to have a yellow card replacement its normally the blind side flanker that leaves the field of play