In the novel, Rocco served under Clemenza, and a lengthy explanation is provided with regard to how Clemenza selected Rocco to replace Paulie.

But later in the novel, and in the first, prior to the day that Michael assassinates his enemies, Tom Hagen makes reference to a "secret regime" being formed by Rocco (this may have been a deleted scene in the movie; I'm not sure). So apparently Rocco got yet another promotion somewhere along the line, altho neither the book nor the movie explain how or why.

In the novel, Puzo writes that after Tessio is eliminated "Neri was sent to take command of the Tessio regime".

So the novel and film go separate ways on this point.

The book would seem to indicate that there were now three regimes: Clemenza's, Tessio's (now headed by Neri), and Rocco's.

But the film, in the "hand kissing" scene, would seem to indicate that there were still only two: Clemenza's, and Rocco's.

What happened to Tessio's men is left to our imagination.