The "hit" on Geary's prostitute took time to arrange just like any hit would take such time. Remember, Geary said that "we had done this before", so obviously there was the opportunity for the Corleones to know about the Senator's habits not to mention that conveniently he apparently frequented one of Fredo's brothel's where he may have been "comped". Because of the time involved, it is probable that Michael had discussed options with Tom before and after the Geary meeting. No, I can't believe that Tom would have attempted to arrange the blackmail of a US Senator involving murder on his own regardless of his temporary "Don" status.