According to scarpa telling the FBI the real reason gambino didn't let Neil kill the whole gallo crew cause mobsters and people would have thought profaci had a handle on his family and could keep them in line. Think about it it was smart cause gambino was still a new boss profaci was around forever and couldn't control some young guys in his family. Scarpa was a real sneaky fuck in the 60tys even rising to capo rite before profacis death. Profaci would have got credit for gambino family taken out the gallos.