As with anything in the clean up crew it all comes down to some level of risk.
Recall your building a system that is designed to mimic the life of the coral reef or a shoal with larger fish or maybe a just one of those small tide pools that has a one fish and some coral in it ( a micro tank)
Anyway you look at it , anything you buy has some risks to it, as each coral , fish , crab has some level of personalitliy to it. Prehaps that crab is use to eating corals , or that fish is timid as it was housed with slightly bigger fish.
As for worrying about buying any creatures for your tank , do some research at the place your buying said crab or whatever. Actually watch the tank , see how they feed them , when they feed them. After you have made a choice, make sure they is some level of return policy for if that creature doesn't work out in your tank.
I know the one place i go has ZERO return policy on marine fish , hence if i buy anything i have to make sure it will be ok. Hence why i don't buy live stock there.
As to the place i do buy my life stock from , they do take back fish and other things if they are not happy in my tank. Although he does charge a little to take it back.