There is no problem keeping different species of Trigger in the same system. As long as it is only ONE of the same species, ie, 1x Titan, 1x Clown, 1x Queen and so on. People have managed to keep pairs of the more aggressive species together, mainly due to different sizes and age, but it is not advised.
For example, Clown Triggers are solitary in the wild. They meet up with a mate somewhere over the reef shelf to do what they need to do. They leave their young behind in relative safety, and go there own ways.
I have to follow the same rules with my Groupers, only one of a kind = harmony. Two of a kind = armageddon.