Is it okay to take the subject matter of a dance, play or song and base the devised piece on that?
No. The purpose of the task is to enable candidates to assimilate the style of the person who wrote the original matter rather than simply borrowing their subject matter. In the case of a song write, the musical style should be assimilated rather than just the words inspiring the song. The technique of a choreographer is what is being studied, not the thematic content of the dance. The way in which the playwright crafts the drama rather than the plot is the central point.