I always have Harlequin Tusk in my large reef. My current 320 gal and my previous 420 gal tanks. My reef is a Mix reef with many small fishes, and a normal complement of reef cleaners. There are a few things that you need to do to keep these beautiful fish co-exist with other beautiful mix reef inhabitants. Harlequin Tusk are beautiful fish, especially the Australian ones. IMO, it is worth it to get the Australian vs the Indo-Pacific. I would recommend that you follow ad vices of people who actually keep HT instead of those just read about them or think that they should act are certain way from their look, size and that they are large predators.
First off, make sure you don't starve them. When they are hungry they will go for little preys in the tank. Mine are always well fed. On the weekend I often stand over the tank and (s)he would come up and beg for food. I feed him bits of fish until he would eat no more. On week days, I feed the tank with a generous portion of Mysis or Ocean Plankton mixed with various pellets and flake in a free for all. He and other fishes would get their fill.
He should be added last if you can. Make sure all the smaller fishes and shrimp are added before the HT. He will not resist dazed disoriented small fishes and shrimp. Put these small fish in such compromised position, they will just be an expensive meal for the predator. If you have to add small fishes or shrimp, add them just right before the light is out (if your tank light dim out for the night, the HT will be in bed long before the light turn dark. I have 12 hrs light periord. This is the best time to add small fish, and add them directly into the rock so they have place to hide and get oriented.
Things that HT will eat:
IME, they will eat any meaty food, no flakes or pellets. Very easy to feed.
Any dazed small fish
small snails, sub 2-3 mm or so. My thriving tiny snails wipe out with in a few weeks. I had many thousands of these freely reproducing snails. All the Cleaner snails are fine. Stomatella and Strombus snails eventually gone from my tank. Tis is due to the combination of my cleaner shrimp (wipe out the stomatella) and the HT ate all the babies snails. Trochus snails reproduced in my tank, but since I added the the HT, no more babies. All the 5mm or larger snails are fine.
I have only a few Hermit crabs and they are fine, no problem. I have a skunk cleaner shrimp and they are fine, no problem. I don't have blood cleanser shrimp but I suspect that they will be fine if you add them before, or use caution when you add them after the HT.
I had anthias, and Royal gramma in the tank in the past but no longer have them. This does not have anything to do with the HT.
Clams are fine. HT will not bother them.
All the corals are fine with HT.
Anemone crabs are fine. I have some tiny small crabs in the rock, came in the LR from Tampa Bay rocks. I removed most but missed a few. They are doing well in the tank for years and continue to do fine. I don't use crabs as cleaner, and no other shrimp other than skunk cleaner shrimp.
Clown fish are fine with anemones. I am not sure about free swimming small clown fish.
I love my HT and would not keep a larger reef without them. Good luck
