"I haven't said that Poles didn't commit crimes againts Ukrainians, Belorussians and Russians," -- and why don't Poles come to Russian forums to talk about that, rather than how they'd been victimized? In general, why are there so many Poles in Russian forums?
"I wasn't able to find any info about it, could you give me a hint about the sources I should look for?" -- virtually any book? It's very common knowledge that the Polish plan for war against Germany was partially offensive in nature (i.e. planned offensive against East Prussia). You can find out about it in virtually any general history of WW2. E.g., Fuller, Tippelskirch... It's really puzzling that you weren't aware of it.