Err... what is _unit exactly?
To get your combatmode line applied to all units within a trigger radius, you could use this:
"_x setcombatmode {RED}" foreach thislist
in the triggers onactivation line (make sure it is a 'present' trigger though).
As for allowdamage, I'm afraid you will have some problems as that command was made obselete (eg. doesn't work) after v 1.0