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.
Hope this helps