Yes Jimmy was a rat, but he did not flip until after he was busted on weapon charges.

CleanBandit, If I am not mistaken, the agent said that to Big Pus in season two. He was an informant as well as Reymond in 95. It was one information against the other, until Jimmy flipped and backed up what Curto was feeding the feds. The FBI deduced that Pushy was holding back on then since 1998 on the true power of Tony in the family at the time.
I forget where I heard it but an CI was responsible for Anastasia and Dimeo going to prison in 95. Febby was an informant in the 1980s. That pretty much just leaves Reymond as most likely the informant whose information crippled the family in 95.