For rays, dimensions are almost more important than volume. I can't say for sure what the various rays need, but most get very large. As for octopi... The short version is "don't".
Long version is that they are too intelligent to be cooped up in a tank, too strong to be contained, and too vicious to keep with anything else.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef