Russia and Poland thing is different - if you would say Slavic and Poland, I would agree. Slavic is the same level as Germanic...
That's not a great comparison- even when Germany is taken to represent more than just the nation-state, it still only represents the High Western Germanic nations- it does not include Low West Germans, i. e.the English, North Germans, i.e.the Scandinavians, or East Germans, i.e. the Goths, Vandals, etc.)
Russia can be taken to represent Eastern Slavs, which means the Russians, Ukrainians, Belorussians, etc, but it's a stretch to say that it represents all Slavs.
Although, you could equally raise this argument for the Celts- there were four separate Celtic groups, and currently it is primarily focused on the Gauls (although Boudicca will add Brythonic representation), but that's balanced out by the Celts' lesser influence and effect on history.
Still, although I would argue that Poland are a valid candidate for a new civ, I'd still say that they're pretty low down the list. Would've preferred them to Portugal or the Netherlands, but I'd also prefer Siam or Abyssinian to Poland.