Actually 'Polish' regions occupied by the USSR were populated mostly by Belorussians and Ukrainians. Those regions hadn't been part of the Congress Poland, for example, though they had been part of Poland before the partitions of Poland that took part in 18th century. When Poland gained independence after the WWI, the Polish-Russian border proposed by the British foreign minister Curzon was roughly the same as the current Soviet-Polish border. The Soviet-Polish war was fought because of that--Polish wanted to have the border more eastwards, and the Soviets didn't accept that (though it has to be noted, that that territory was declared to be a part of Poland when Poland gained independence, not as the result of the war). Anyway, my point here is that the Poles were a clear minority in those regions, the majority of the population were as ethnic Soviets as it is possible to be ethnic Soviet (i.e. Belorussians had own soviet republic as had the Ukrainians). And, these territories were an old claim by the USSR (and Russia), the territorial claims weren't invented by Stalin to expand. Though expansionism was the reason for why these territories were eventually occupied--but that shouldn't make anyone surprised.
On the other hand, I don't have any idea if there was significant partisan activity in the Polish regions occupied by the USSR. If there was, they shouldn't be national provinces for the USSR. (The USSR may gain cores for example after the WWII as the new borders slowly settled down.) If there wasn't much partisan activity, I see no reason why they shouldn't be Soviet cores from the very beginning of the game. At least from reading quickly some stuff from Wikipedia, I got an impression that the Soviet occupation was rather peaceful, at least when it comes to active armed resistance. But on the other hand it would be better to have a more 'scientific' source.
