a clean up crew like cmaxwell39 said is all the inverts used to eat left over food and algae, although a lot of people do not like putting the hermit crabs in there because they will kill your snails for their shells, ive had 50+ snails die because of this and i had around 200 shells for them to use scattered all over the sand but i guess they wanted the snail shell, even tho after they killed it they tryed it on and decided they didnt like it and went back to their previous shell
i would get nassarius and astrea snails, maybe some nerite
mandaring dragonets and scooter blennies both eat pods as a main source of food, they will starve unless you have a decent sized refugium, and 100+ lbs of rock, if you do decide to get one i would pick one of them, not both as they will be competing for the same food, even if you find one at your LFS that is eating frozen, it will starve if it does not have the pods for its main diet.
you could get a shrimp/goby pair like a yellow watchman goby and a pistol shrimp, or a jawfish if you have a 3+ inch sandbed
i would get rid of the crushed coral and switch to aragonite sand since crushed coral will bring nitrate problems its fairly cheap if you dont buy it live, and then buy just a little bit that is alive to seed your tank.
it looks like you are doing the research before buying everything, which is a good thing, i have had bad luck with impulse buying
lol
so have fun and i cant wait to watch this tank grow, keep us posted
also what type of equipment do u have?