I think that tony biggest mistake was when he didn't got invovled in the conflict between little carmine and john, he should have taken carmine side and help him against johnny.
I mean johnny is smart and have big ego, and that make him dangerous.
And carmine is more a peacefull guy and and stupid who could be easily manipulated when needed, so tony should have taken is side so carmine could be the boss and thourgh him tony would control ny too