What sorta "rules"?
One of the things i thought was weird as an observer is that we end up judging all the plants together and peeps get butthurt and start complaining.
Like how do you judge between a 14 foot tall monster or a beautiful weed bonsai?
Maybe have a few lil "goals" or "competitions" like biggest cola, best indoor and a best outdoor.
Another thought is to just do a communal grow, keep it organized and such. That could be an alternative to a competition.
Im in either way, let me know what i need to do and when i need to start!