The trigger could be part of the problem, but i'm still not convinced that its the whole problem. I could see if you added one fish at a time and he chased it constantly around your tank until it stressed out. Does he go at them the whole time, or just when they get close to his area? 90g should be enough space for them to get away and get a break from him. If you really think its the trigger, before you add your fish, change your rocks around and mess his area up. Also add them with the lights out. It will put your trigger into night mode, and he will also be looking to claim a new area to call home, which should help take his attention away from your new livestock. Do what you can to give the new fish a chance to settle in before they have to deal with it.