I think that the Viking/Norse core area should be shrunk a bit. You should not have stability penalties because the Russians built St. Petersburg...
I have not yet played this newest version of game, but actually one reason why St. Petersburg was built was to destabilise that area and make change of geopolitical situation in favor of Russian Empire.
And that was very successful move from Tsar Peter and one of reason why Swedish Empire start to slowly collapse. I use that tactic often when I play. It is big gamble to build city in border of rival territory and make it one of your most powerful city - but if it is successful you almost always see rival civilization to collapse. In real history Russian Empire and Tsar Peter took even greater risk and moved Empire capital to St. Petersburg.