“whacking the heads of the families” simultaneously, was so secret [until needed to know] even Tom was not told, neither Clemenza nor Tessio....

“cooperation from inside the families” would have been quite an operation – one little whiff and catastrophe

Among others,

1. who would have been the go-between
2. whom to approach and sound out
3. whom to trust not to go squealing to their boss
4. what happened to these people who cooperated Rewarded or got whacked themselves!