Mostly, it's hit or miss. All octos are different, so even though one of us could have great luck with stars or slugs etc., another person may have a completely different experience introducing a tank mate.
I had stocked my octo tank with black foot trochus and hermit crabs prior to my octo's arrival, thinking they would be food at some point.....but they are all still alive and he pays no interest to them. However, some people on this site could tell you their octos have eaten plenty of hermits.
A few months ago, I did introduce a chocolate chip star to the tank. My octo was clearly not thrilled about this five-armed moving thing, and spent the first 48 hours harrassing it and trying to pry it off the tank wall....could have been curiousity, I'm not really sure; but they live together just fine now.
Make sure to research the animals you are interested in keeping in your ceph tank - some can give off toxins when stressed or dying which could ultimately wipe out your whole tank, others could hunt your octo [Fish are generally not a great idea for tank mates]. But always keep in mind, whatever you put in there could end up being dinner for your octo one day, so I'd recommend not spending too much!