Just a suggestion, but I think it would be a good idea to provisionally rank everyone and spread everyone across groups and/or playoff tree (whatever the correct name for that is). Not give everyone a rank number per say, but spread everyone about by what ladder they are in. So all the diamonds aren't grouped together and similarly all the bronze. Then if you go with the double elimination format that's already been suggested with no group stage, this will give a chance for the less skilled players to play against people of a similar skill level in the losers' bracket, assuming they lose in the winners’ bracket. This will also keep the tournament interesting for the higher skilled players. If there is going to be a group stage, it will give players a chance to play against players of a variety of skills as well.