their are some small dwarf species but they are hard to identify so you could very well just receive a baby octo that will grow to be much bigger. if you receive a genuine dwarf then it will be most likely full frown and have only weeks to live...hardly worth it if you ask me.
on the other hand...
im in the process of setting up my first aquarium and i was wondering weather i could bypass accasonal under gravel cleaning if i bought a sand sifter starfish...keep in mind i plan to use about 1 1/2 inches of sand for substraight