Where these originally prisoners of war (POW) formations like the Czech Legion?
More like the Latvian Rifles.
That is Poles inside Russia, for whatever reason, wanting to support the new independant Poland.
I wouldn't characterise the Czechs as "POW" formations though. Many were there from other sources, including many who were "POW" only in the most technical sense, having actively deserted. They started fighting during the Great War.
It is the German, Austrian and Hungarian "Internationals" that were largely POW. The clue is that they did not appear until the war with the Central Powers was over. Only then would they agree to fight.