I concur with a lot of what's said here. Clown triggers are often very mean, but I have seen exceptions. I totally agree with the undulated triggers being a risky choice, and nigers being safer.
HOWEVER,
When you're dealing with smart fish they tend to have a lot more complex behavior and individual personality (e.g. triggers, cichlids, angels). Triggers are one of the smartest families of fishes (they have a highly developed telencephalon). When someone asks me if a trigger will work in their tank I tell them the same thing I tell people who want to add a cichlid to their cichlid community, or an angel to their coral tank: It might work, but I can't guarantee anything because it depends on the individual. I don't know the temperament of the fish you want to buy or the other fish you have, so I'd recommend trying a safer trigger, and if it doesn't work out then return it to the store.