The Corleone Family chart in the Senate hearings scene lists Fredo as underboss at the time of part two. Obviously that position was ceremonial because Fredo had no business being an underboss.

As for part one, I doubt Fredo would've been listed as such, I don't think there was an underboss at that point as things were in a bit of disarray. Santino was underboss to Vito, but Santino was now dead and Michael had taken the family from Vito so I don't really know how to answer your question in regards to Part I...

Neri was a caporegime in Part I and Part II as well as being Michael's personal doberman.