I've obtained the ebook version of The Sicilian and I've been looking into it. It provides some interesting details on Peter Clemenza and I don't think it's been mentioned here before.

*According to the Sicilian Clemenza was "the chief underling of Michael Corleone's father back in America". Clemenza's brother Domenico asks Michael in front of his brother: "Does Don Corleone really set him on his right hand?". This implies that Clemenza was in fact acting as Vito's underboss after Sonny's death.

*The Sicilian also strongly implies that Clemenza was born in the province of Trapani as his brother is the Capo of this region.

*Every five years Clemenza would travel to this region to take with him about twelve new recruits for the Corleone family in New York, basically "Zips". These are men that have been handpicked by Clemenza's brother, Domenico.

*The Sicilian also states that Clemenza commands over a hundred men, so it is safe to presume this would be about half of the Corleone family's size since Tessio commanded the other half.