I'm gonna try to hit a few of the questions.
1. sand sifting stars:
If you have a DSB
this is a really bad idea. They feed on the in fauna that keeps your DBS functional. The infauna is what shifts the sand around, ingest sand wich removes bacteria and clears the way for new bacteria in addition to being a minor food source for some corals and fish. Any sand sifting fish or invert will decimate the in fauna population. Sand sHifting critters on the other hand are fine. They just move the sand as they scoot across the surface. They are actually considered beneficial because they keep the aerobic areas of the DSB
supplied w/ oxygen.
...not reef safe.
3. Linkia Stars. These stars need a minimum of 100lbs of LR
to have an adequate supply of food. They also require high quality water and a very slow acclimation. If your tank is under 6month old or you have less than 100lbs of rock please do not get one of these. The chances are much greater that is will not make it, than that it will.
I would suggest the Red Serpent stars or the Red Feathered stars..They are not nearly as aggressive as the green brittles.
The formia...I dont know anything about them.
Hope that helps