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!