The US version of Sing has only three songs, right? (Charm, Silence, Sleep) Does this differ from FF4 or FF4A? One of them has Curse, I remember reading. Also, most guides say that the song differs depending on the harp, but for FF2us I can't tell a difference. The songs/chance of songs seemed the same.
As for other characters using the command, they need a harp equipped, but it seems to work fine. Cecil seemed less successful than Edward in the song effects -- the only stat he has that's lower than Edward is Will, so the chance of success might be related to that.