Of all the French new wave films I've seen so far, Les Cousins did appear to be the most mainstream, or perhaps a better way to describe it was that it was the most conventional, as far as the standards of classical Hollywood go. The only major stylistic deviation comes in a scene where Charles gets in the car with Paul, who shows him around Paris. There is a rapid succession of shots from the POV of inside a convertible driving along the Champs-Elysées. There is no logical order to the cuts, and you experience a sense of giddy disorientation you feel when looking all around that area of Paris for your first time, trying to take in all the sights at once. This scene seemed to me to be a precursor to the sequence in the beginning of Godard's "Breathless" when Jean-Paul Belmondo is driving and then shoots the police officer.
One of the major themes of the new wave that is present is the blending of high and low culture. For example, during the party scene in Paul's apartment, he insists on listening to classical music while everyone is getting drunk and fights break out. While the yé-yé music craze was still a couple years off, Chabrol could have used some other sorts of music more "appropriate" for such a party such as jazz, but instead he chose Wagner.
There was something else I noticed that Godard touches on in Breathless (and in a handful of his other films before he went into the extreme experimental/political realm) and that is the attitude that love is for squares, or something to be avoided altogether in favor of casual relationships. When Florence falls for Charles, Paul and his slimy friend (whose name I can't recall) essentially browbeat her until she gives up the notion of being in a relationship with Charles, and she goes on to have a fling with Paul. Belmondo's Michel Poiccard also seems to lament the fact that he has fallen for Patricia when he tells his friend Berruti, "What's worse, I think I'm in love with her" to which he replies "Damn!" You could read this either as another example of how the French new wave rejects the conventions of the classical Hollywood love story, or also perhaps as a rejection in general of their parent's generation's ideas of love and relationships.