I might add I can't see Michael asking the Molinari Family for murder favours - The Corleone pride like the Corleones can't handle their own hits....

Sure thing it would always be "someone from within the family" even if a civilian!
Remember! civilian Michael was sent to kill Sollozzo and Police Captain McCluskey

In fairness, everyone except Tom [even so Tom was seemingly still kept in the dark of Vito and Michael's Baptism murder plans] “was considered a risk to turn after Vito's death” until Tessio outed himself as the traitor

Clemenza has to answer for Paulie since Paulie was a member of Clemenza's regime and no clearance for Clemenza until establishing Paulie was acting on his own “for Vito's shooting”