I keep a brittle star (who spends most of his time hiding under a rock, as Oliver gets grumpy and tries to pull him around the tank) and two damsels. Most people advise very strongly against damsels, as they have been known to attack octo's eyes, but these guys seem okay to co-exist, and have done for two months so far.
Anything you put in with an octo that might be snacked on, will be. I had a very pretty (and much larger than Oliver) shrimp that vanished mysteriously one night. I'm waiting for the damsels to start disappearing, but apparently fish are friends, not food.
More seriously, there is a pretty short list of things you can keep with an octo, and even some corals are on the verboten list. Try looking back over the archives - a lot of people have discussed the things you're asking in a great deal of depth in the past.
i tried keeping a clownfish with mine......the clownfish was lunch! no more nemo lol. i had mine with some soft corals and i had a few starfish. but i have heard people keeping octos in a full blown reef and everything was fine but i def. wouldn't risk it.
You want an overflow with ALL of your equipment in the sump. You will need a good filter rated to 1.5 times the amount of water volume you have, a REALLY good skimmer rated to 1.5 - 2 times the amount of water volume. you NEED to have enough live rock. A heater or cooler for a bimac.
It appears that you are going for an octo (Cuttles are better )so the live rock can be set up however you want it really, it just HAS to be secure, Ive even read an article on someone buying the marine glue and gluing it all together, but that seems a bit drastic. If you do go for a cuttle (beter choice ), many people like to pile the live rock on the sides, or one side of the tank to give them an open "play area" because they tend to swim more than octos.
I was thinking of getting a drilled tank off glasscages.com and setting up my own sump and refugium... lol can I ask why cuttles are better : D? They seem to be harder to come by and harder to care for?