But it seems to me that, when the civil war was raging, all the while the Germans had the Russians by the balls (and Lenin recognised this, see Luxemburg on the subject), a few of those Polish battalions who'd been fighting alongside the Tsar's Russian troops prior to the revolution, went over to the other side of the civil war.

Maybe this is just a legend, but I don't think it is. In any event, the people (well, person really) who ran the Soviet Union after things settled out of that civil war had a long memory in addition to being a homicidal paranoid maniac.

Looks like it could be / should be at least truish. During world war one Germany set up a puppet state, Kingdom of Poland. When tsarist russia collapsed, the polish troops probably did not feel themselves obliged to serve a new russian government and might have either fought under a white general or joined (or attempted to join) the puppet state. It would at least fit the general pattern in that war zone at the time.

While checking around to see if there was a polish equivalent of the Czech Legion I found this sobering quote:

A total of 2 million Polish troops fought with the armies of the three occupying powers, and 450,000 died. Several hundred thousand Polish civilians were moved to labour camps in Germany. The scorched-earth retreat strategies of both sides left much of the war zone uninhabitable.