according to the aquarium info dropdown it's a 55 gallon being converted from freshwater to saltwater.
Kudos for doing the research first. You're saving yourself a lot of money and headaches doing that.
I'd add the inverts first (drip acclimated). I would hold off on the diamond goby for awhile. They do an awesome job keeping the sand clean but that is also how they eat. In a new tank they may starve. It can be a lot of work making sure they are getting enough to eat. The tang, clowns and gramma should do fine. The gramma may pick/pester at the crabs. Keep it well fed. Definitely recommend
live rock and a good protein
skimmer along with powerheads for water movement. I don't care for the wet/dry filters on a
saltwater setup. They are good filters but I don't really use a mechanical filter on my saltwater setups.