Nah allies of Cross Guild
The shadowy organizations supporting them
Well, it seems the ones supporting Cross Guild are the Underworld Emperors (Du Feld, Peclo, Morgans, Umit, Giberson...).
To me, these ones are not very related to the Underworld Emperors, but instead they're nobles, corrupted kings, mercenaries, assassins or... Even Knights of God.
Take note that Caesar Clown said that there were two pacific countries interested into Shinokuni... And now we know that the Knights of God are the ones who punish those ones who do not obey the World Government orders/don't pay respect or tributes.
So, yeah, I think they're lurking "in the dark" like dormant agents until WG knocks at their doors.