I forgot to touch on the future plans for your tank. I would try to avoid the Cucumber till your sand has been established for at least a year or so. Same goes for the Mandarin; one year minimum for your tank to mature. Also, in a tank as small as a 54gal, I would avoid putting a Cucumber and Mandarin in the same tank. The Mandarin will need a heavy supply of live pods to survive, and even though the Cucumber will mostly clean up detritus and stir the sand, it will also be consuming some pods on its way around the tank. I don't feel that a 54gal would be large enough to support both animals at the same time.
You also mentioned an Angel being added to your tank. I have had 3 different ones in my 54gal, and they have been my favorite fish that i've ever kept! My personal experience includes a False Lemonpeel, Rusty, and Bi-Color Dwarf Angels. The best one for me was the False Lemonpeel. It was always out and about in the tank, never picked on a single coral, and almost completely rid my tank of algae! I brought home a Rusty Angel to give that one some company, and it ended up that the Rusty had a fungus which ended up taking out both angels. Now I have a Bi-Color and it's not really the best fish. It is kind of skittish and doesn't really take much care of the algae. Also, it picks at my Kenya Tree and Purple Anthellia quite a bit. They are both very resilliant corals, so it's nothing that I worry about, but if it starts picking on anything else it's outta there!
The following Angels are listed in the order of what I would consider the best, down to not as good which would be suitable for your tank:
Lamarck's Angel - Safest around corals and pretty hardy
Japanese Swallowtail Angel - Poor shippers, but hardy after they become settled.
Masked Swallowtail Angel - If u don't mind paying, they are a great fish.
Atlantic Pygmy Angel (Cherub Angel) - Very hardy and safe
Flameback Angel (African or Brazillian) - Same as above, but just slightly less hardy and about twice the cost
Most other Pygmy Angels will usually be good for a reef tank, but you do run the risk of them getting a taste for corals. It's not uncommon for any Angel to have a taste every now and then, but most likely that is all it will end up being. Some of them however, will take a bite and then never stop! Avoid any of the cryptic Angels - Golden Pygmy and Potters - as for some reason they do not have a very good survival rate.
I think that's about all I can think of for now. I am a huge fan of Angelfish, and they have been a great addition to my tank. Let me know if you have anymore questions on them and i'll try to help however I can.