It's tough to tell sometimes without seeing the deaths or even having a body to examine, but a few possibilities come to mind.
1) Aggression! You got quite a mix of fish there, including some that are notorious for being tempermental (namely the damsels). It is also usually recommended not to keep multiple pygmy angelfish together because it is a gamble if they will fight. If most of these fish were in at the same time, this is my best guess as to what happened. Either direct aggression or the stress from it.
2) Bioload! You mention you tested your parameters and they were fine, did you specifically check ammonia and nitrites? That is a lot of fish in a new tank, it could have been too much for your biofilter. What kind of filter/pump do you have? Do you have any liverock, livesand, etc. or just a filter/pump?
3) Oxygen levels! This is in conjunction with bioload. Saltwater holds much less oxygen than freshwater....so it is important to have surface disruption to maximize oxygenation. Depending on the type of powerhead and size of the filter, this may have been a problem.
Once again, these are educated guesses, or things for you to investigate further. Hopefully, you can find something that you can tweak to improve your tank. Your enthusiasm is great, so we just have to isolate any potential problems and get them to fall in line. Two additional issues I see are:
1) Bioload! I think your tank is fully stocked right now and would not add any more. The tang will get big, and room will be at a premium. Some will say a YT cannot survive in a 55g, but I would say you are on the fence of possible success/possible failure for long term success with the fish. Time will tell if you have no plans of upgrading. Provide some liverock and open swim spaces, as tangs need ample swimming room but also rock to graze off of.
2) Anemones! If they are Caribbean species, it is important to know what species. If you don't know, pictures would help us ID
them for you. All Pacific anemones require fairly intense lighting for long term survival. This would include either multiple Compact Flourescent lighting or Metal Halide units. If not provided, the anemones will not make it.
Good luck, information can be a whirlwind so keep asking questions!