Cycled =/= mature. Mature tanks have an incredible diversity of microorganisms that simply are not established in a new tank. New tanks go through many waves of blooms of different animals while this sorts itself out, creating a lengthy period of ecological instability. Mature systems are what "reef safe" stars need as it is, in theory, the diverse fauna that they feed on (although some Echinaster species were recently found to be at least partially filter feeders! Shows you the state of knowledge about these). The only real exception to the maturation time of a tank is if you take rock from a mature tank and just transplant it into a new glass box without disturbing it much, which will give a much shorter stabilization period - although the sand bed will lag behind in development.
If the orange star is a Linckia or Fromia, by all means try the silversides, but don't be surprised if they fail. These stars are usually characterized as "microfilm eaters" - which means they feed on who knows what on the rocks in large, diverse systems and people don't understand it very well. I have read everything from sponges and Tunicates to bacterial slime and cyano being eaten by various stars in those genera, but nothing clear-cut enough for hobbyists to avoid having starved stars with alarming frequency. People often suggest target feeding with other, large foods to avoid starvation, but none of them ever tack on an actual observation to back up the suggestion. Again, I strongly recommend you give this star either to another experienced hobbyist who has a very large and mature system with a lot of biodiversity or take it back to whatever store it was gotten from before it reached you.
Not to meddle in personal affairs, but, if it were my tank, I would also have words with the person who handed me the bag with the star in it. Family members, friends, and significant others can be very enthusiastic about aquariums, but when the success/failure of a system is not on their heads and wallets, they tend to be a lot more care-free about it than they should be. In the marine world, an uneducated enthusiast can easily leave the LFS with a bag containing a dangerous and/or toxic animal (some fish, some sea urchins, cucumbers, etc.) in addition to those that just have an incredibly poor captive care record and should be left to experts.