If the aircraft is certificated by the manufacturer as a single pilot type you can fly it single pilot if you are appropriately rated. For public transport operations (certainly in JAA land) all turbojets have to be operated by two typerated crew so you can only fly single pilot if the aircraft is being operated privately. If you are flying an N-reg on an FAA licence and you have done the single pilot ticket you can fly wherever you like privately single pilot (two out of three, airspace doesn't matter). I don't know of any authorities that insist on two crew for private operations of a single pilot turbojet type.
Incidentally, the 12,500 weight limit doesn't matter if the aircraft is a single pilot type - you can operate a CJ3 single pilot privately if you wish.
Hope that helps
Tom