The question of how the peace between the families and Mike was worked out is a tough one.

The book and movie make it look like they other living capos's from the other families were so afraid of Mike they fell in line. Again this is fiction so it could have been that way.

If we put it in a real mafia context, Mike would have reached out to friendly capo's in the other families and nade accords with them beofre the hit to ensure a smooth transition of power.

Not unlike what Barzini tried to do with Tessio.