Yes, you should use add_ai. The problem that you are having is that the default enemy AI cannot see very far in a nebula, so they generally ignore everything.
Unless you clear it and build a stack from scratch you don't know who will be the leader. All you really need is one line per ship:
<add_ai name ="F73" type ="CHASE_PLAYER" value1="15000" value2="12000"/>
<add_ai name ="F62" type ="CHASE_PLAYER" value1="15000" value2="12000"/>
<add_ai name ="O35" type ="CHASE_PLAYER" value1="15000" value2="12000"/>
This would cause them to chase Artemis if it comes within 15000 meters in open space or 12000 meters in a nebula.
"Damn the torpedoes! Four bells, Captain Drayton!"
(Likely actual words of Admiral David Farragut, USN, at the battle of Mobile Bay. Four bells was the signal for the engine room to make full steam ahead).