"This tournament is cancelled if it would be Double Elimination due to number of players."
Livin' on the edge, right? Right after a barely 4-players tourney...
Yes, its a bit of a gamble, but the theme would be messed up by double elimination, as players get new set of elements when advancing to next round.
Also, from 6 players, we can do Single elimination, so the danger is for 4-5 players only.