Rio Negro is on my bucket list. i would go to Larry Dahlberg's webiste and get his take on what tackle to bring, I would be willing to bet that guy has caught as many if not more peacocks than any other resident of the U.S.
He is pretty quick to answer questions.
I did a fly swap with a guy in Florida this winter, he sent these pics shortly after.
So, I still haven't got to fish for them... but at least I have tied a fly that ended up stuck in a few of them!