In regards to the starfish you wish to keep:
"Individuals of Linckia species, particularly Linckia laevigata, are profoundly and seriously harmed by rapid changes in salinity; additionally, they appear to suffer "shipping" stress. As a result these animals need to be treated VERY carefully during acclimation to the home aquarium. This acclimation should be done slowly, and; acclimations of more than six to eight hours are often required. Once established in aquaria, Linckia generally
appear to do well and may persist for a year or more. However, they often seem to slowly die, probably due to a lack of some specific dietary item." (taken from
reefkeeping.com)