The bottom line is triggers, with the exception of a couple are territorial fish. As small fish the have more space in the tank, and do not feel the need to make more space for themselves, and can live in a community environment for some time with no issues, until they get large and decide to "clean house" to free up some space. Even the "peaceful" niger that is often considered a good community choice will be fine for years, when it gets large 12-18" it will be a completely different animal. Triggers also have alot of individuality, so there will always be the few exceptions to the rule out there. However with size comes aggression, just like people say their 3" undulate trigger is the wimpiest fish in their tank, ever heard someone with a 10-12" undulate say that?
I by means consider myself a trigger expert, there are many here with more experience with these fish than me.