Unfortunately you would be combining predators with a natural prey item. Most octo's that are available would be small enough for any shark or moray to eat. If you did get a larger octo it would probably be near the end of it's life cycle since they only live 1-2 years. Serious ceph keepers get their pets as juveniles, depending on the species, with the head equivalent to a golf ball at largest. This way they ensure some time with their pet. Even a larger wild caught octo. would be very uncomfortable locked in a tank with Sharks and Morays. Ceph's are unimaginable intelligent and interesting enough to warrant their own quarters. My Bi-mac (octo) pulls apart my Mag float 350 cleaning magnet and pushes it around the tank for fun. I had another octo that would crawl just above the tank and jet water at me from across the room to get my attention to be fed. They are easily as smart as a dog. I just recieved three cuttlefish (Sephia Officanilis) . This is a European species which is almost impossible to get. It took me all of two days to train them to eat from my hand. On the third day they were already lining up at the glass to be fed like puppies and these are just babies. I have noticed they are beginning to do things to get attention to be fed first. For example flashing colors and undulating the colors over their bodies so it looks like one of those scrolling digital signs with the messages on them. That is really cool to watch! I will let you go for now since I could ramble on for awhile with out stopping. Good luck with your decision.
After my wife has the baby and things settle down I might hold a meeting. Right now everything is Hectic as I work in the excavating and Landscaping industry. I don't have pics but if Greg comes over again and has a digital camera he is more than welcome to post some. I don't have a digital camera and have never posted pic's before. Maybe in three months or so I could host a meeting.