"South Pacific" opens a nine-week run Feb. 27 in New York City Opera's second Musical Comedy Season. The Rodgers and Hammerstein show, at the New York State Theater, will have alternating casts. Susan Bigelow and Marcia Mitzman, both of whom have appeared on Broadway, will alternate as Nellie. Opera singers Justino Diaz and Stanley Wexler will play Emile de Becque, Muriel Costa-Greenspon and Camille Saviola will alternate as Bloody Mary and Richard White and Cris Groenendaal as Lt. Cable. Actor Tony Roberts will be Billis and Paul Gemignani will conduct.