To answer the original question, Rocco served under Clemenza until he became a capo himself at the end of Part I. After the hit on Paulie, Rocco became Clemenza's right-hand man, so to speak. As has been said, the novel has three regimes after the "Day of Reckoning":

1) Clemenza (same as before)
2) Rocco (secret regime so Barzini and Co. wouldn't know it existed.)
3) Neri (took over Tessio's regime, later becomes Head of Security when the family becomes "legit")

Sonny's regime was, if I recall, broken up and the men re-assigned to Clemenza and/or Rocco, although I don't remember if this is actually stated in the novel (I think Tom says this when Michael tells him that "[he's] out".)

Now the film has just Clemenza and Rocco as capos by the end of Part I. Neri is the "special enforcer" and doesn't exactly have a regime (although he probably has the authority to command buttonmen). HOWEVER, I'm certain that after the Lake Tahoe hit Tom asks Michael about Rocco and Neri's men, so that would imply that Neri is a capo, albeit also the "Chief of Security" for the Family.