Jihad conspiracies has the first really in depth discussion of the belters, including a bit about teh supposed existence of "immortals." while JHS Terra has a more, mundane description that goes into more detail--and is easier to introduce if you don't want to risk the impact of effective immortality might have.
If anything, the Belters resemble, in outlook, most liberal 21st century states, while the rest of the Inner Sphere is closer to the 19th and early 20th Century colonial empires--which is to say, the belters are more or less insular when it comes to conquering their neighbors, as compared to the Inner Sphere, which is functioning on a zero sum, colonial style foreign policy.
If you want to use them, though, you get into the problem of--why? What does the IS have that belters want?