2. Control Panel » Group Management » Site Permissions > Set Total # of private topics allowed to 0 (zero) to disable the PM function.
So users with 0 - 50 post give them a group, automatically asign them to another if they post more than 50, gve the 50 + group permissions to store 50 messages or so.
4. Yes
5. Add a few lines yourself in "Control Panel » Display Options » General > HTML includes Default header. Find a piece of PHP source to do so.
6. Yes
Other questions can be answered by others.
Rob