I don't have a trigger fish but I have and do keep aggressive fish (puffers) and other fish that typically don't do well with other tank mates. The thing is to find what uh, triggers that aggression and curve it.
A lot of times it is just being in a small tank (compared to the ocean), cramped with other fish. How would you react being confined in a 20 X 20 foot room with dogs, cows and other people? I'd think your captors would come one day to find that you've completely demolished everything in your space too. So it is with these fish. They have a lesser tolerance than others to be confined. But, as some have stated, it is possible to keep the fish with other species peacibly. And that is the solution you should search for.
I believe a "happy" fish is a peaceful fish. Try adding other elements to your tank in the form of borders, hiding places... ENTERTAINMENT! Yup, to curb my bronze puffers from chewing on their tank mates, I added snails. Now they spend most of their time using their brains and advanced eyes to hunt the crunchy critters. Since then, none of the other tank mates have been harrassed or nipped, in over a year.
Or, what's on the menu? If you put a tiger cub in with a calf, what do you think will eventually happen? But if you keep that tiger well fed, the calf will have a chance of survival. So while most of us tend to keep our fish well fed in our opinion, sometimes it's a matter of the fish fending off competition because they are not getting enough. Or maybe they are lacking a vitamin. (Have you ever craved a food item?) Diversity of diet was the solution for one of my fish and more of it. Since there was enough food, the competition stopped. The tank needed to be cleaned a heck of a lot more, but there was not comp.
Then there was the Tilapia. It's trigger was being alive. He stopped harrasing the fish when I added butter and a frying pan. (Ok, I didn't eat him...)
But find the trigger for your trigger and you'll have a nice tank. Being alone isn't always the answer for the fish. It's a solution for you. Try to make your fish happy.
I should be the Fish Whisperer!