I wasnt talking about breaking the Omerta, but changing loyalty.
Something like when someone saves your life and you are in debt with them. Or in families where the Omerta isnt for the entire life.
So, maybe is like you said, and no matter the situation isnt appropiate to change sides, or maybe there isnt a protocol to follow with the pacificatori with this matter either. But if we know for sure about this we can already understand if some characters will be allowed or not to be with us, and that way without expecting someone to change sides and follow us when that doenst make sense in the world.
That would depend on what actions are being taken by the individual, regarding "changing loyalty."
Is the person making a formal oath (like what MC did with Blackie in that "Promotion" event), making it permanent or just helping out for a single, one-off time?
If it is a one-off thing (as you said, paying back a debt), well.....I'd think a debt can be paid without one actually changing their loyalty to their existing Family.
Something permanent (i.e. formal Oath) though, I'd say would likely be a huge "no-no" even if an individual becomes a 'made-man' (like Blackie said).
There may not be a rule said out loud about it yet, but my assumption is, a made-man released by the Family (with no longer any obligation to work for the Family), is still expected to not work for other Families and is expected to just live out his life as a civilian (and working for rival or enemy Families especially I'd say would get them the death sentence more likely than not).
If we're taking your example with Fawkes per say, traditionally him being in a same team with the MC is highly unlikely. But there could be circumstance or possibilities around that.
Either Feni Family somehow has a huge fallout with the Bonanzo Family, forcing the Feni Family to seek another Family for asylum (yeah, sounds a bit similar to Eiza's arc, I know), or Fawkes' father could die and Fawkes becomes the new head of his Family, and decide to ally with DeLucas.
So possibilities can exist, yet it all depends on the circumstance/situation changes.
And traditionally, I'd say such act is probably hugely frowned upon, especially if it's permanent and not a one-time thing (to pay back a debt for the sake of the honour/respect).