I'm not sure what reg actually applies to this but I do know that unless you are flying a 501sp or 551sp you have to have a single pilot endorsement from flightsafety the FAA or other school. It doesn't make sense really. I used to fly a CE-501 single pilot all the time but I couldn't do in our CE-550 which is basically the same airplane. I guess it is just another way for flight safety to pry more money from all those flight depts.
From what I remember about six years ago, the 501 & 551 are certified under FAR 23 while the 500 and 550 are FAR 25 airplanes. Same nuts and bolts, but the part 23 certification only allows a mtog of 12,500#, which can be limiting on the 550. I got a single pilot exemption from Simuflite, this requires an anual checkride in the airplane with the FAA. The airplane also has to be certified, certain things have to be installed such as Id swich on yoke, headset with boom mike, and a few others. I think most of them came standard equip on most citations. It seems like the exemption required that you not exceed the 12,500# limitation while operating single pilot.