Nog, I'd say your imagination of Gondor is more or less correct. There were the descendants of Dúnedain and the descendants of the old people from White Mountains, definitely some minority of people with mixed Southron blood (in Pelargir & co. probably larger), not to speak of people with elven blood in Dol Amroth (now the Prof himself can tell us something about half-elves being uncanonical
), some Rohirric people, either migrated or just young Rohirs (from the Edoras-Minas Tirith au pair program), and from the time of the Kin-strife also some people with Northerner blood. And they all lived together in what seemed like a peaceful state (no inner problems).
Of course the picture (and the more the comparision to the RL) is a little bit idealised, but still I believe the main point is there.