Yes, my theory about Neri trying to nose out Rocco is just my theory. But it's based on more than a 4-minute scene:
--Rocco was Clemenza's choice; Neri was Michael's. Another guy who wasn't Michael's choice--either as a brother or as a consigliere--was Tom Hagen. Look how he was treated in II.
--At Anthony's party, Rocco is only a security guy; we see him doing nothing but surveying the crowd. Neri's nowhere to be seen.
--Then Johnny Ola shows up for his meeting. Neri introduces himself to Ola--he's a player. Michael turns to Rocco and says, "Johnny's men look like they might be hungry. Why don't you take care of them?" Now he's a waiter as well as a rent-a-cop.
--Deanna gets out of line at dinner. Rocco tells Fredo: "Michael says that if you can't handle this, I have to." Now he's a bouncer.
--Then Michael has his critical meeting with Frankie in the boathouse. Neri's with him. Rocco is nowhere to be seen. Presumably putting Deanna to bed--perhaps he's holding her drunken head over the toilet.
--After the Tahoe shooting, we see Rocco galumphing around looking for the perps. Where's Neri? Maybe sleeping? He doesn't seem part of the security detail.
--And when Michael returns from Cuba, he asks Rocco and Neri to leave the room so he can talk alone with Tom. Rocco goes right out. Neri starts out, stops, wheels back, swipes a cigar from the table, and then goes out. (Cheeky litle advantage-taker, don't you think? )
--Then there's that penultimate boathouse scene. Another clue in that scene: Rocco stands, says nothing; Neri slouches in a chair, leads the discussion.
--And of course, Rocco goes out on the one-way mission; Neri survives and is still alive and kickin' 20-odd years later in GFIII.