Hybrid/overlay will assign two (or more) potentials
to the same atoms.
Hybrid will assign only one. You can use the * * syntax
to assign one potential to some atoms, and another to
other atoms. You need to use something like
Fe NULL at the end of one and NULL C on the other.
You can't assign AIREBO and EAM to the same atoms
with hybrid. That would be hybrid/overlay.
The doc pages for the pair styles explain all of this.
Steve