I'll answer what I can. Pretend you numbered your questions to make it easier for lazy me
1. You can use GCN controllers for some wii games, but not others
2. You can have people use GCN controllers or Wiimotes. Unfortunately, the GCN controller is much easier to use than the wheel mode wiimote, so the person with the wiimote is at a disadvantage
3. It depends on the game. Nunchuck is used in some multiplayer games and most singleplayer. Even in Wii Play, Tanks uses a nunchuck. My favorite wiimote Smash Bros configuration uses a nunchuck. Probably a couple of Mario Party minigames uses one as well.
4. No idea
5. No idea
6. It's a good idea because it doesn't waste batteries. Up to you, though.
7. You can never ask too many questions