good rule of thumb, don't add something to your tank until you know exactly what it will do to your tank. I would avoid collecting anything from close to shore because of pollutants and disease that can easily enter your tank and kill everything. Specimens collected far from shore have a better chance of being healthier (just think of all that people and boat traffic close to shore, oil and gas leaks, pee, chemical dumping, cosmic dust, global warming, oops, sorry, I got carried away - you get my point :mrgreen: ).
With starfish, the longer the legs (arms, whatever) and less "webbing" the better chance it will not destroy your tank (think batik star and chocolate chip star versus my favorite linkia starfish). Good luck, hope this helps.
-Dr Marco :sfish: