Blue linkia sea stars are reef safe, tho very difficult to keep in captivity. They eat algae that grow on rocks and most reefs are kept too clean to support them. Sand sifting sea stars are reef safe. I have never had either last in any of my client's tanks or my own.
Depends on what you want to have Reef Ninja. If you want mostly snails I would say 1 snail per gallon and maybe 30 crabs with 5 emerald crabs for grass algae and green bubble algae. Now I got lucky with my starfish. I have a purple starfish and a sand sifter and they do well with the others in the tank. I also have 2 shrimps one cleaner and one fire shrimp and they love to eat left over algae in the tank along with any foods I put in take. I hope this helps