The Solomani Movement (which may have been more benign in its early stages) seems to have started around 600, and rapidly gained influence in the aftermath of the Civil War.

If anything, it seems to have arisen in the Core, rather than than on the Rim.  In that sense, parallels can be drawn between the Solomani Movement (and Party, and Autonomous Region, and Sphere, and Confederation) and the IRL Zionist Movement, which had its origins with secular Jews living nowhere near a "homeland" that now was thoroughly alien in the outlook of its population.  

Until the Solomani Movement was granted de facto control of the Autonomous region in 704, I suspect that the Rimward Sectors would have been thoroughly multicultural: there is no way the 10-odd-billion citizens of the Terran Confederation are going to culturally (and genetically) overwhelm a few sectors of well-established Vilani.  The Core Sectors would likely have been more resolutely Vilani in character, especially of the common people, even if the elite classes were more Solomani-ish in their self-identification.  Surely they would have exaggerated their distinguishing "Solomani" characteristics to differentiate themselves from the plebs they ruled over ... even if those exaggerations bore no relationship to the contemporary culture of Terra and the Rim.

Also IRL, I note the well-documented example of, say, the Greek population in Australia.  Their self-identity as Greek-Australians hearkens back to a idealised image of "Greekness" from the late 1800's, which then gets frozen in time and passed down through the generations in that cultural community in Australia.  When a Greek-Australian then visits Greece for the first time, it can be a shock to note that habits and outlooks that define them as "Greek" are very outdated compared to how modern citizens of Greece see themselves.

Aaaand, just arrived in my intray is Bruce's reply, which I agree with.  The Nazis were aiming for a German monoculture.  The Soviets were acutely aware of the multicultural nature of their state, and national identification played a significant role in intra-Soviet politics.  Similarly, published materials (esp. DGP's Solomani & Aslan) emphasise the confederal nature of the Solomani state, and the need for the central government to juggle the priorities of the many (often powerful and conflicting) member-states.  It makes it much easier to paint individual Solomani patriots (and even the Solomani state as a whole) in a more sympathetic light than "Nazis in space".