I have a red serpent in my aggressive tank. No one pays it any mind. Picked it up to help with the clean up (leftovers) and it does a good job. It hides during the day and comes out when the tank is fed. Tank mates include picasso, valentini, SFE and a volitans.
Good Luck!
Just though i'd add..
I just got a Green Brittle Star today, he is great!
My dwarf lion pays no attention to him at all, though, that is it for fish in my "aggressive" tank atm..
I tried a brittle star in my aggressive tank . It got lunched buy the triggers. I then added a large choclate chip star, well the triggers ate the chips off him but since then they haven't touched him. Guess They had their fill , Hopefully