A couple of standard issues, and a pet peeve:
1) IMO, you need a large tank over 100g for a Linckia, and for best long term success with a Fromia. Fromia are well known for dying at about the 1 year mark, most likely due to starvation. A large tank with a lot of LR is, IMO, absolutely the best road to success with these guys. A few months or a year should not be considered success. Only add one of these stars to your tank for best success. They do not exist in high densities in the wild, and need a lot of algal and bacterial films (not macro algae, hair algae, etc) to survive. Wouldn't you eat a lot of bacteria to survive!? It also takes time to regrow in an area that the star has passed over. A Linckia will readily reproduce in a large healthy system, which is the only way, IMO, to add another seastar to such a tank.
2) The reef safe stars (excluding brittlestars and serpentstars) are algal and bacterial film feeders, including other encrusting organisms in their path, though the exact diet is unknown. They will not eat pods, or any prepared foods. Do not rely on spot feeding. This will not work.
3) Though they may eat coraline algae, most people report (as with urchins) that the coraline grows back quickly in a healthy reef tank. However, these animals are not raspers as urchins are, so I am not certain that they really eat that much coraline algae.
4)All reef safe stars are very delicate, and need a large, healthy and mature system that is not prone to water parameter fluctuations. They need a very slow acclimation, at least 4+ hours, using a drip method. This is important for brittle/serpentstars also. The ideal specific gravity is normal reef level- 1.025-1.026. I can not emphasize the need for proper acclimation enough. Death by acclimation stress occurs within a month of introduction or stress. After this, the common cause of death is starvation.
5) The green brittlestar is a known predator in the wild, and this behavior has been reported in reef tanks. While the damsel is a likely meal, my green brittle has not touched snails, or the damsel even though he has grown significantly. Sometimes they are trouble, sometimes they are not, but they will happily chow down on anything dead, dying or ill. Brittle and serpent stars must be spot fed and should not be left to scavenge. I do not recommend the green.
6) Though I know it is not a popular statement, I hope you will let me say (as I study brittlestars) that a basketstar should not be kept in a tank, because successfully keeping them will result in their ultimate death. Basketstars are a very long lived animal (at least 7 years), growing to over 3 feet across when expanded, and need far more than 2 guppies a night. This is not a normal diet (when have you seen a guppy in the ocean?), similar to feeding asian freshwater carp (aka goldfish) to lionfish. How much plankton do you think a basketstar eats when it positions itself on top of a nice coral, in ideal current on a reef? Lots! Even if it is growing, when are you going to get a 3ftX3ftX3ft min tank? I've said it before, and I'll say it again, please do not get a basketstar for your tank. They are best left in the wild. They require, at the very least, hand feeding after dark.
7) The "purple Linckia" is often not, it is often Tamaria stria, which may or may not be reef safe. Another one where some people have trouble and some don't.