The peppermint shrimp is a cleaner shrimp. Besides looking cool they will post themselves high on a rock and wait for fish with parasites to visit them. The fish will circle then swim slowly on it's side signaling the shrimp to climb on. It's cool to watch the shrimp investigate the mouth, body and gills of the fish searching for parasites. Once the shrimp has established itself well in the community the other fish will protect it feircly (look in my other post for the line "trigger fish bite").
Crusteacean need ALOT of calcium to grow hard healthy shells. See post about Kalkwasser and corals.
DO NOT GET A SEA CUCUMBER!!! Thet look cool but if they die they release a severe toxin THAT WILL KILL YOUR TANK.
Snail of all sorts will eat the sea grasses that will grow naturally on the live rocks. They are detrius eaters too and help clean your tank.
With such a small tank you really need to watch the amout of animals you put in it. A rule of thumb is one inch of animal for every three gallons of water. Hence, you can have 6 or seven inches of animals. Freshwater is 1:1 ratio. Now you have a 1 inch goby, 1 inch Damsel, two half inch snails and a 1 inch peppermint. A total of 4 inches of animals. Be patient and don't buy any more until your tank has fully cycled. Refrain from doing major water changes until it has cycled (meaning the ammonia has reduced itself to nitrate then nitrite). Use your test kit regularly.
OK I got sidetracked. If your animals start to disappear look for a mantis shrimp. Get rid of ti quickly. It will eat everything. Regular crabs (other than hermits) will to. Avoid clams until you can afford high dollar lighting. Actinic won't do. Don't try to grow any type of animal that has ??zoozanthel?? algae. This includes corals, anemones, and many other soft bodied animals. The "Z" uses sunlight to produce sugars to feed the host it lives in. Metal halide or halogen lights are the only types I could consistently grow them in. Most crateceans don't care about lighting. Feather dusters are a welcome addition after a couple months and don't count against the fish to water ratio. Starfish will eat almost anything if they are hungry. Use a straw to push a dead guppy or other food up under its arms twice a week.
I can't beleive your dumb enuff to read this whole rant LOL.