I wouldn't worry about whatever money Guettel has made from Piazza. Whatever it is, it's not even cab fare compared to what he must have inherited from his grandfather's billion dollar estate. That's why he's always lived quite comfortably (to say the least) despite the fact that his composing career has been far from lucrative up till now.
As far as singing composers, don't forget Noel Coward who performed in many of the shows he wrote, as did George M. Cohan. Irving Berlin had a charming, but very tiny little voice. Marc Shaiman and Scott Whitman are fun performers.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
[http://margochanning.broadwayworld.com/]
"The Devil Be Hittin' Me" -- Whitney