Theories aside, Rurik shoudn't be Swedish/Norse by the game mechanics. Or most of the Rus will be Sweden/Norse after several generations via conversion.
There should be at least melting pot for creation of Russian culture.
And well, I also think that Rurik story is just a myth.
Yeah, pretty much this, except I don't even think a melting pot event is necessary. Just make Rurik Russian and have that be it, otherwise things will, like you said, turn into Swedish Russia. He could rule over Ugric-Baltic or Finnish or whomever it is and have the cultural conversion events bring in Russians, or maybe just his capital could start Russian. Something like that would probably be more appropriate than some overly complicated way of making the Rus appear.