The Trigger has probly already claimed the tank as his taratory, thats why it is not recomended that you add triggers first. How big is the Trigger? I hope you know that you will have to upgrade sometime in the future. Put as for now, a nice Eel would work well, but no puffer and trigger in a tank that small, to much potential for problems. When you add a fish, now that you have that trigger in there, make sure you do it at night, when you see the trigger either sleeping. Also changing rock formations when the new fish is added will help, because then the trigger wont know the landscape and will think he is in a new spot, he has not yet claimed.