Based on experiences journaled, you will likely receive an octopus in the abdopus complex and most likely an adult aculeatus.
Yes, it is a good idea to remove the clown as the maroons are particularly territorial and may be aggressive toward the octo. The anemones could be a problem depending upon what kind and their size. A large anemone can trap and kill a young octo but it is rare to have one large enough to harm an adult. However, most of us do not keep anemones because of their sting and the possibility of causing skin damage that can lead to infection. The snails may or may not be eaten but can be left in the tank. The urchin and brittle star are good clean-up crew for an octo tank.
Neither brine nor freshwater fish should be offered as food. The brine have no nutrition (as well as being too small and will polute the tank) and the freshwater fish (in addition to not providing appropriate fats) are very often treated with copper and can be fatal to an octopus. We recently had a member loose an octo after
feeding goldfish. Whether it died because the fish were treated (as the keeper now thinks) or whether they created an ammonia spike is not known but the animal died an unpleasant death the next day.
Most of our octos have readily accepted raw frozen shrimp but not always immediately. You will need to be sure the shrimp you are getting are raw (many of the frozen packaged shrimp are cooked). Starting with a few hermit crabs and one or two other small live crabs (fiddlers tend to the be universally accepted food) and snails is a good idea. Hermits and snails may be eaten initially but will likely be left alone after the animal is accustomed to other food. You can also try raw clams, oysters and scallops. Mollusks can be partially opened to temp the octopus but be sure to remove any uneaten portions very quickly. We have found ours will take them once or twice but then loose interest. The brittle will compete for the opened mollusks but there still may be uneaten food that should be removed. Some form of live food should be offered at least once a week, small crabs are the most accepted and usually the easiest to obtain.
The only freshwater food that should be considered as a treat are shrimp and crayfish. Keep in mind that the freshwater animals will not live long in a marine tank so you will need to remove anything not eaten if it dies.
