I have 3 ruby brittle stars that came in as hitchhikers. They are very small and stay in the same place Will they eat fish and will they knock down rocks as they grow larger. Thinking about isolating them.
In general, it is reef safe. If you have corals that are not glued down or encrusted to the reefscape, large serpent stars can knock them down at night when they are active. That would be the only issue.
The only serpent start you need to worry about is O. incrassata (Green Brittlestar). Large specimens are known to actively hunt for fish.