Sometimes I miss the feeling of what it was like to be a beginner, with the entire realm of the saltwater hobby in front you of. Unfortunately this hobby has a way of quickly giving us a reality check, so here goes!
Am I correct that this is your first attempt at a marine system? If so, you should know that the majority of the livestock you named is not livestock that you will want to attempt until you have a lot more experience. In freshwater you can pretty much keep anything , so long as you have the correct equipment and tank size. In saltwater you need EXPERIENCE. This is a vague concept that is hard for a newby to understand, but every experienced marine aquarists understands exactly what this means and why it is important.
Lest look at the list.
Candy can shrimp: nice choice for you.
Crabs: depends on what you mean. MOST of these are not compatable with corals and inverts. They are opportunistic predators and will eat pretty much anything they can catch. You should stick with the small Hermit Crab varieties, such as Red Legs and Blue Leg Hermits.
Brrittle starfish: No way. Starfish are extremely difficult in any system, and your tank is to small to have enough diversity of micro life to support a starfish's dietary needs.
Sea slugs: Near impossible to keep, even for the advanced hobbyist.
Clams (any dwarfs?): Not for you. Clams are the for highly advanced reefkeepers only. Dietary needs and lighting needs are very specific as well.
Anenome: Also extremely difficult. They are often purchased and rarely live longer than 6 months in captivity, despite a natural life span in excess of 200 years.
Shrooms: Great beginner coral
Tube worms: Controversial. Some say they can not thrive in captivity, others report success. They are inexpensive and readily available, with no special light requirements. If you decide to give it a go, be sure to feed it with some of the smallest micro sized liquid foods. I personally do not keep these, having accepted that they just don't live in captivity.
Other good beginner corals will include Green Star Polyps, Yellow Polyps, Button Polyps, Zooanthids, and some Leathers. (Beware Leather corals grow fast and LARGE!)
As for the 20 gallon cube for $500. This is a horrible deal in my opinion. These setups are for people who want instant gratification and are unwilling to go through the learning process. The equipment is not what you would naturally select, and the price is very high.
I would start your aquarium project with this deal:
40 pounds dry rock & 40 pounds aragonite sand $105 from 40 Pound box Key Largo Rock, <br>40 Pounds Bahamas Aragonite Sand<BR>pay shipping on rock only - KL40-40
This will give you the rock and sand you need for a tank up to 29 gallons in size, at a great discount below retail. For lighting, I would look into compact florescents. You can find some nice fixtures on eBay at a great discount. This is a beginner system, so there is no reason to overboard with costs or fancy equipment. You are just getting your feet wet until you upgrade to a much larger tank in the future.
For a skimmer, you will use a simple hang on model. This should be a significant portion of your budget. You have to realize, the skimmer is the MOST IMPORTANT purchase you will make. It will make or break your success in this hobby. For you, I suggest Super Skimmer with Needle Wheel - Up to 65 Gallon | Venturi Models | Protein Skimmers | Aquarium - ThatPetPlace.com