I'd imagine only the GB and fighter could attack the trader, not the destoryer. I see nothing wrong with a destoyer in the group though to say fend off enemy ships of larger caliber that may try to stop the fighter and gunboat.
What I want to know, is it true if a transport mounts gunboat equipment is it no longer safe from destroyers+
Also, can you hi-jack a freelancer ID if they are 30+