Well, there's plenty to know about this fish

. I'd suggest doing a RC search on them during low traffic hours.
For basics, shrimp are likely just going to get eaten. Small fish may be eaten. Snails and hermits may be eaten slowly, but you'll be able to replenish them as they're depleted. Good news is they're completely coral safe!
Most are "gentle giants", but some are aggressive and get more aggressive with age. I've actually heard they all start out as the stereotypical nice tusk, but as they age they get meaner. Mine has gotten increasingly aggressive, but he was moderately aggressive when I got him, too. He never had the usual shy behavior at first. The first thing he did was claim the tank as his own... Just be prepared. If you're planning on having smaller fish, add them first. If you're planning to get larger fish, add the tusk before them because some people have had their nicer tusks get bullied to death.
They're crustacean eaters, so offer it prawns. It should go nuts for clam, squid, and just about anything. Even fish. They love meaty foods. I'd avoid krill because feeding too much of that has been linked to lockjaw- a fatal condition.
Your 120g is a good tank size for one, so no worries there. Six months old is good, too.
They're pretty hardy fish. Be sure to get an Australian tusk, though. The Indo-Pacific tusks are cheaper, but you're taking a risk. Those tend to be caught with cyanide and die shortly. Australians have more color than Indo tusks and aren't caught with cyanide, so the higher price is worth it.