Well, I have a 200 gallon tank that has 3 returns in the bottom of it, and I have seen lots of tanks that use returns and closed loop drains in the bottoms. I did it for asthetic reasons...so I could have the tank look as clean and uncluttered as possible.
You are right, I left cephalopods out completely. I never really gave much thought to keeping them...probably because I know very little about them, and don't know where you can obtain them here in the states. I never see them listed for sale at online vendors or local shops.