Answer:
The rules in rugby are pretty wide on this - I have directly quaoted the IRB rule 3.3 WHEN THERE ARE FEWER THEAN FIFTEEN PLAYERS
"A Union may authorise matches to be played with fewer than fifteen players in each team.
When that happens, all the Laws of the Game apply except that each team must have at
least five players in the scrum at all times.
Exception: matches between teams of Seven-a-side are an exception. These matches are
covered by the Seven-a-side Variations to the Laws of the Game."