forget about the octopus, they need a large water volume and feeding is an issue.....best to keep with something that is relatively easy to maintain, can support itself for a few day on its own without feeding. I think that a dwarf angel of some sort and a gobie/pistol shrimp pair and maybe a fairy wrasse or gramma would be great, colourful and lots of action......that a few interesting inverts and you would have an interesting group.