Hi Dariusz,
thanks for going out and trying to establish content partnerships. I
think a license like CC-BY or CC-BY-SA would be fine if the content is
clearly labeled as such. The problem with CC-BY-NC or similar
"noncommercial use only" licenses is not necessarily incompatibility,
but that they don't fit our established definition of free content, i.e.
far too many uses we want to allow become impossible.
All best,
Erik