Micheal kills all the heads of the other 4 families. So Micheal should be capo di tutti capi (boss of all bosses). If he was a capo di tutti capi he could do the same thing Maranzano did.
Micheal arranged a meeting.
Micheal Corleone began the meeting by explaining how this new organization would run, that he would be the boss with five specific lieutenants below him (setting up the 5-family structure). These lieutenants would be: Peter Clemenza, Fredo Corleone, Sally Tessio, Rocco Lampone and Al Neri.

Is it possible?

'This was just another Bronx tale.'