I'd say no to to the green brittlestar. First of all, there is a great size mismatch. Even if the brittlestar didn't harm your octo, he could steal its food (As I recall, Colin had some problem with this.)
Then I consulted Anthony Calfo/Rober Fenner's Reef Invertebrates, which has detailed information on almost all the species you'll encounter (and wonderful photos, too!) The green brittle starfish attacks small fishes, but it's also know to attack shrimps, bivlves and quite a few other animals. It has quite a strategy for catching the little fish, arching up on its legs and making a nice, safe looking cave. When a fish comes into the "cave", the brittle star drops down on it and eats it. This species of brittle star grows quite large (over 12 inches) and is recommended only for tanks having large fish, not for tanks with invetebrates or smaller animals.