Greg,
I snuck that reef photo in. That is not my octo tank. That is my nano-cube. The greatest reef tank setup I have ever worked with.
In the octo tank Ziggy is in a 'crib', a small plastic 'critter cage'. In the tank I have 1 damsel which is very mild mannered, 3 brittle stars (1 small, 2 baby stars), 1 peppermint shrimp, 1 sand-sifting star, 2 conch sand sifters, 1 chocolate chip star, 1 turbo snail, 1 small female fiddler crab and a small workforce of blue and red legged hermit crabs.
I have a couple of pieces of live rock that I hand-picked for there 'caves'.
The bottom of the tank is 2/3 fine reef sand and 1/3 fine crushed shells.
Lots of the tank livestock will become food for Ziggy once he outgrows his crib. I need them now to keep up the tank and give the biological filter a load.
In the nano-reef-tank I keep anemones but only because the clowns like em. They are a big pain and a risk to the tank. Also, they need light that I doubt the octo would be happy with.
My previous bimac left my damsel alone for the most part but I had one damsel that would try to push the octopus out of it's territory! If you have a good healthy anemone that is established I think you would be 'ok' but I would think it an unnecessary risk. Don't let the anemone tumble down on the octo's lair.
Do you have a new tank for the octopus? If so you could get it cycled up fast by using rock, sand, filter material, water changes and other stuff from your cycled 150. It is expensive but I am a BIG fan of Fritzime. It is the only product that delivers what so many other flim-flamers promise.
Otherwise, get a plastic cage to put in your tank for a crib and see how it goes.
Nancy,
Thanks!
I am keeping the clams in a 2.5 gallon glass aquarium right now. I plan on putting the whole 2.5 gal in my 75 gallon. I will keep glass lid on so I can feed the clams snow without it dispersing in the tank. Someday I will let him sneak into the clam tank and have his pick.
He seems to have a hard time opening the clams so I cracked the shells of two today and gave them to him.
You might try buying a live clam or mussel, opening it and cutting a tiny piece for Zig and seeing whether he would take it. He can't eat baby clams forever. 
