My friend's friend is a trigger maniac, I guess. He has one Undulate Trigger, three Picasso Trigger, one Clown Trigger, and also two Damsels, altogether in the same tank. So I guess you can put an undulate trigger with other fishes.
About a trigger being tame, I don't know for sure, but my friends has a Picasso Trigger and it's in a way tame. It killed cleaner shrimp and crab, but those are invertebrates. And everyone knows that trigger and invertebrates are not the best combination. Anyway, since my friend bought it, she never feed it anything but brine shrimp, probably that's what causes it to be tame. If you feed them life food, for example goldfish since they're little probably that'll be different story, because I once read that once a triggerfish bite something alive (other fishes), it'll keep doing it. But I don't know, probably that only applies with Picasso Trigger.