Hi - I haven't been to respawn before but am looking to come to this upcoming one...saw this thread and thought I may be able to help with some competitive TF2 insight

If you're interested in seeing what tournament rules are used in Australia, these are the official rules of the main league:
http://www.ozfortress.com/showthread.php?t=265993G, 3H, 3I, 3J and 5A are the key sections of interest, a lot of the rest is irrelevant for a LAN tourney. All our 6v6 competitions use rules the same or very similar to these (i.e. mirrored maps are played first to 5 instead of 6 on the CyberGamer ladder, which is also more common internationally).
I'd recommend against turning off class restrictions in later or any rounds. It's really more about making a positive impact on the fun, balance and flow of the gameplay for all teams (especially good teams) than ensuring a noob team does or doesn't get rolled (it won't make a difference either way) - so in that respect, having limits like 3 of everything and 2 medics in a 6v6 game won't do anything since good teams wouldn't run more than that anyway. The stricter limits above squeeze a more interesting variety of classes out and really make you value your medic and use that ubercharge wisely. And aside from the way it affects the flow of the game, let's be honest...medic isn't a terribly interesting class to play and it's one more person who gets to play a combat class, and the person who does play medic will feel more important.
The core three competitive maps are cp_granary, cp_gravelpit and cp_badlands. cp_fastlane has been seeing more play since respawn times were adjusted, while cp_well has kind of been on the way out (not on the map schedule...don't know if it will return). cp_dustbowl was once played but has been retired for a long time. Other official maps aren't really played at all. There has been a custom map league featuring maps such as freight, yukon and follower. Some are okay, some are crap. As Kale said though most people probably wouldn't know these if you ran them.
Control point maps are generally the only ones played competitively, so I'm not sure what scoring rules would be best if you ran payload or arena. You'd probably want the same class restrictions though.
Hope this helps~