GF was based on the novel. Moe Green appeared in the novel, but Roth didn't. The novel ended in 1955, and so did the film. At the time GF was made, no sequel was envisioned (Paramount was fearful it'd be a flop). But after its smashing success, they decided to make II, and to flesh out the story, Roth (based on real-life Meyer Lansky) was added, as was much else.
I think Hyman is more of a Jewish name. Remember Roth ws born around the turn of the century when people had different ideas of what was a normal and approriate name. There are lots of Italian names in the films that are no longer popular. Such as Luca, Fredo, Rocco, Genco, Amerigo, and even Vito and Santino.
Well remember that Frankie "5 angels" was also added to the story as was several other characters. If you have the VHS version of the Godfather Trilogy, you may have the “A Look Back” behind the scenes commentary about the film The Godfather II and FFC tells us that most of the scenes of young Vito Corleone were from the book, but the story of Michael during the 50s was something he invented. So with that in mind there were several characters erected from this, including Frank Frankie Pentangeli, Hymon Roth, Johnny Ola, the Rosato and several other characters with smaller roles in the film.