The bonfire has been posted on the Quidditch Pitch, including a three legged race that I'm sure will provide the opportunity to enter the "awkward/likely to end in disaster" part of the writing contest.
The rules would be the standard rules you would expect for any of those events.
In terms of the egg and broom race, it's like an egg and spoon race only with the egg balanced on the brush of the broom. If dropped, it must be placed back on the broom of the person who had it - i.e the transfer must be completed successfully before the next person in the relay can go.
In terms of writing rules, standard site/Quidditch rules apply. It's considered bad form to claim you won outright but instead give a suggestion of how well you feel your character did, in terms of what goes for them and against them - for example being a fast runner, having worked well as a team, but only being younger students and therefore having shorter legs, or having taken time to get into the swing of it.