I'm surprised people thought he would be Rooster- though obviously this may be nothing like the musical, I thought Cannavale didn't really sing. If that's the case, and Easy Street remains in the film, I'm glad he isn't playing the part. He's right for it type-wise, but I don't really want to see someone barely making it through one of the best songs in the score.
I am a firm believer in serendipity- all the random pieces coming together in one wonderful moment, when suddenly you see what their purpose was all along.