breaking ppl up randomly is a horrible idea.

probably 5v5 with gamemodes such as capture the flag or capture the point would be nice but not sure if local populace is familiar with such things at all (since afaik not many ppl play fps anymore and even fewer tried anything further than dm)

you should try a tf2 tournament instead. the tf2 scene is growing up nicely. lots of new clans popping up to contest with the old. check out the gamesync forums. they're still exploring the possibilities but most rules have already been established locally and internationally.

breaking up people playing randomly = opportunity for group of friends to game the system = splitting the prizes between the friends = waste of time for organizer
hope that makes sense as well.
if you don't know what gaming the system means then yeah, you probably should just stick to whatever is easier to understand since that works for you.

good luck.