pygmys could but u want a 30g for them, due to waste, normally. I think there called balis? not sure. But they come from the wild so you might find yourself in a sistuation where your octi dies after a few weeks becuse tyhe animal is already an adult. I believe there life span is only 6 month from hatch to death anyways.
THere are several pygmies that can be kept in a 10-15 g system, but most are fairly secretive and nocturnal. We often keep O. bocki and O. wolfi (which is the more difficult of the two). Both only live a few months as adults and probably have generation time of less than a year. O. mercatoris is a bit larger, but can surive in a well cycled 10 g. They are fairly easy to get from Florida gulf coast suppliers in the spring when they breed. We also keep Hapalochlaena lunulata and other small blue-ring species in small systems, but again they only live as adults for a month or two and, of course, they are dangerous.
I assume these are kept in flow-through systems and not in individual tanks with their own filtration - it seems that the tank can be smaller with flow through. Interesting collection of octos you're keeping.
As I recall, you're quite good at photography, too. Maybe you could post a pic or two someday.