I have answered similar questions on this topic in the past, particularly with regards to early medieval Spain (links below). Most of the 'barbarian' groups that established post-Roman successor states numbered only in the tens of thousands. Large enough to affect serious political changes, but still an overwhelming minority in terms of total population. However, I would not say that the new rulers remained a distinct and isolate group. Identity is a messy thing, and is more tied to culture than genetics (though inter-marriage did happen). While early medieval law codes might stress the legal distinction between, for example, Frank and Roman, over time the lines between identities blurred and blended. It's a bit of a broad topic, but if you have any follow up questions, please feel free to ask!