Octos do need something to live in. Virtually anything is suitable that they can get inside of. I've found them in paint cans, bicycle frames, swim fins, shoes, you name it. Ceramic things are a good choice, flower pots for instance. Fishes, other molluscs and just about any crustaceans are in some danger.
Most folks keep octos by themselves but there are exceptions.
Most octos will adapt to frozen fish or shrimp as a primary diet. That crab will never know what hit it. Octo's don't really like fish that much but they will take adult guppies or goldfish as a pleasant change from a steady diet of frozen fish or shrimp. You should plan on feeding your octo at least every other day. Octopuses have comparatively short lifespans.