Just have the rules laid out beforehand and decent refs and it should be fine. Also you could try and decide how many people will be participating by creating the thread and then deciding the team sizes once you get a good number to go off of. You also have to think about sub sizes. I think a 7v7 would ideally have 3-4 subs. However, if you did a 7v7 then you might be limited to only 2 subs if you want more than 5 teams (basing it whether or not you can get 40+ people to be interested).
Also I haven't checked the forums recently or played. Can someone fill me in on what happened with the recent tournaments?