Ok..you know how to feed them...but will you have the space to keep them? I know that they like large amounts of open space, even juveniles. And if one gets too big, will you have someplace to keep it? It seems that they get prettttty big as adults, and I'd think they'd need almost cownose ray level requirements of space (which is a lot)...
I know that choc chip sea stars are more beginner stars, so that's good, but will a linkia be able to do well in the tank?
Macros, you'd be better off asking in the macro/mangroves subforum. I know that C. prolifera would be better than seagrass though, since seagrass needs 8"+ of substrate, and the horseshoe crab would most likely dig up any seagrass you had.