ive heard that you can keep certain urchins and britle stars with octo, can you keep sand sifting stars with them also? (without them eating them)

yup sand sifting stars will work. Octos dont eat echinoderms

But avoid big jaggy urchins like pin cushions as they always jag the octopus

I wouldn't recommend a pencil urchin, either - nice urchin in the right place, but mine has eaten a lot of my coralline algae and doesn't really help much with the tank clean-up. He's pretty, though, and doesn't bother the octopus (except that sometimes the urchin's path leads into the octopus's den!).

Nancy