Allegro (musical) - wikipedia article, gives lots of good background info and links to other sources that may be useful in your research
Speidel, S. E. (2015). "Gods of the theater, smile on us:" elements of the greek chorus in the musicals of sondheim, stephen as influenced by rodgers and hammerstein's "allegro." Available from ProQuest Dissertations & Theses Global.
Internet Broadway Database - lots of images of the original production & rehearsals
Weiner, A. (1980). The function of the greek tragic chorus. Theatre Journal, 32(2), 205. [Article about the use of the greek chorus. May provide you some background info and understanding of the importance of the chorus as used in Allegro.]