Yes, the primary reason was to make the scene more dramatic, more violent. To add to the drama, FFC used a variety of killing methods that seemed a bit illogical. Neri, acting alone, kills Barzini, a bodyguard and a chauffeur with just a standard police-issue six-shot, .38 revolver. But earlier, Rocco and an associate use M3A1 "grease gun" submachine guns to kill Tattaglia (and the hooker he was in bed with). Dramatic license, I suppose.
The probable reason that Cuneo and Stracci didn't die in the novel was that they may not have taken a direct role in any violence aimed against the Corleones--but Tattaglia and Barzini did. Michael may have spared Cuneo and Stracci because he figured that, having killed the other guys, C&S would be scared enough not to cause any further problems. He may also have needed an alliance with them to cement his role as undisputed boss of NY Mafia, and thus to pass on the title to Clemenza when he moved to Nevada.