lol SkarmCressBliss. The only one out of those three that's even used anymore is Blissey, I think. Yes, OU is used for standard battles, although there is a UU ladder.
Anyway, rules that are in place for battles:
Sleep Clause: Only one Pokemon on either team may be asleep at a time (excluding Rest, I think).
Freeze Clause: Only one Pokemon on either team may be frozen at a time (impossible to enforce on WiFi, however)
Species Clause: You may not have two of the same Pokemon on a team.
Hax Item Clause: Items that rely on luck, such as Quick Claw, are banned.
OHKO Move Clause: OHKO Moves, such as Sheer Cold and Fissure, are banned.
Evasion Clause: Moves that raise evasion, such as Double Team and Minimize, are banned.
And the one that nobody uses:
Item Clause: You may not have more than one of the same item on a team.
There could be more, but those are the main ones, I believe.