That really depends on just how pico you go. With the extremely small tanks IMO there aren't any suitable animals with the possible exception of a tiny snail. If you go to the over 1 gallon range you could then go with a snail or two, a hermit or porcelin crab, or maybe a sexy shrimp. There are no suitable fish for a pico IMO and there are some hermits and snails that should be avoided. Only the smallest inverts may be suitable. The idea of a pico is to see how small of a tank you can keep corals in successfully, cleaning is usually done manually without the assistance of a cleaning crew. If you want to add them for movement or interest then do the animal a favor and set a larger tank up.