Hi there! Ink has lots of cohabitants! A serpent starfish, lots of hermitcrab snacks, many bristleworms, who I have noticed are much more active now that there's an octo in the tank again! And some weriod looking snail. I know many others have lots of different creatures in with their octos! Just nothing that can hassle them.
I don't have live corals, so I just have a plain florescent fixture. My tank would take 2 of these, but I think it's more comfortable for Ink to have just the one, so he has dark spaces to go to. Although he chooses to live in the cave at the front of the tank so he can watch us!
I have a Forbes Sea star in mine. Any echinoderms are fine...stars, cukes, urchins are fine too as long as there not too spikey. As far as lighting, try to use as little as you can and still be happy with the way it looks. I think mine is around 20-30 watts on a 30 gallon and thats plenty. Remember a powerful light will also heat the water up.