I admire Pistone, but I am as sure as a person can be without proof that he did cross that line to ingratiate himself with the Bonannos.
I have many reasons to hold that view, including the fact that the movie Donnie Brasco depicted him as present at the 3 capos hit and taking part in the disposal. Pistone said most of the movie was pretty much accurate.
Which parts were poetic license? You decide.