My tusk is around 6" and is very active and outgoing. I would consider a hiding Tusk to be a sign of stress or still acclimating to it's territory depending on how long it's been in the tank. Also, the presence of more established aggressive fish might also cause a tusk to be more withdrawn. Despite a tusk's "mean" appearance and size, I find them to be somewhat passive and not nearly as aggressive as some tangs or triggers. I would be hesitant to add a tusk to a tank that has an established clown trigger and niger trigger already in it, as those fish could easily keep the tusk stressed out. Of course, tank size, frequency of feeding, and individual personalities of the triggers and other fish all can factor in. In general, I prefer to to add more passive fish first and most aggressive fish last, as this allows the passive fish time to acclimate and establish territory without being stressed by more aggressive tank mates. As it's been mentioned before, I would not have another wrasse with the tusk, as that can definitely create issues.
I have never heard of not keeping other wrasse with a tusk. Never. I've heard that you cannot keep 2 tusks together, but not with other wrasse. Tusks, even though they ate a wrasse, look nothing like what most wrasse look like, the elongated narrow body. So they usually don't mind them because they're most other wrasse are very dissimilar to a tusk, burrowing, ect....
I have owned two tusks in 20 years. First was an Australian housed with a Lunare wrasse in a massively overstocked tank and had no issues. The tusk was also in there with a Clown Trigger, Niger Trigger, Picaso/Huma Huma Trigger, Panther Grouper, Volitan Lionfish, Dogface Puffer, Porcupine Puffer and a Long Horned Cowfish.
This was in a 72 bow, my first tank 20 yrs ago, and didn't have the internet to say how big these fish got. It was a common thought that they will grow to the size of the tank.
Anyhow, no issues at all between the fish.
My last tusk, that I had to put down a few months ago, was the Indian Ocean version. He was housed with a full grown Melanarus wrasse, a Red Coris juv, and a Formosa juv with no issues and the tusk was there first.
But every animal is different. And a large Clown Trigger may be more aggressive towards the tusk. Depends on the fish.
The only triggers I wouldn't keep a tusk with is an Undulated Trigger or a Queen Trigger. Way too aggressive, even for triggers.
But yes, if the tusk is hiding, he's not doing well. Usually when they are caught at a large size they don't fare well in captivity. It's much better to buy a juvenile with the eyespots still visible.