Packed from end to end with faultless performances, Phoenix Theatre's Ruthless! The Musical was a stunner. Joel Paley's book and lyrics — about a little girl who'll do anything to play the lead in the school musical — provided a convulsion of nudging and winking at anyone up on their backstage stories. But a cast made up mostly of Phoenix Theatre veterans (including Debby Rosenthal, whose send-up of Mama Rose as a clueless stage mother who winds up a star was a scream, and Rusty Ferracane, in a glittery black skirt and eyelashes out to there, singing about stardom) nearly vanished from sight whenever little Riley Glick appeared onstage, singing and stomping about as 9-year-old Tina. Glick's way with a scene-stealing brat was surpassed only by hearing her shout an expletive early in Act Two. And her ballet combination wasn't bad, either.