Well, a 55 gallon tank comes in standard dimensions, so unless you spend the cash to get one custom made in the dimensions you need, you will probably have to look for something other than a 55.
A 55 is too small for a blue tang, even a small blue tang. They have different requirements than other fish. The swim constantly and have a very high metabolism, so they need a long tank to go back and forth. They also are very sickly and are disease magnets. Definitely not a good choice for a tank that size, unless you don't mind them spreading disease to all your other fish.
Canister filters do not work well for saltwater tanks. You are better off just getting a high quality protein skimmer. A protein skimmer in conjunction with live rock will be such better filtration than any canister filter. Canister filters tend to lead to poor water quality over time, which will make it very difficult for you to keep inverts like urchins and shrimp alive. Urchins and shrimp do not tolerate high nitrates, and that's exactly what canister filters tend to produce.
Oh, and do go with sand substrate. It's by far the best option.