were those measurements in inches or centimeters???
Blue rings do not make the best captives, but are certainly interesting. Safety wise, you would need to have a tank that you never need to put your hand into...so a sump is a must. Dwarf octos can be very aggressive, and frequently will rush out from rocks to bite the unwary octo keeper.
Probably, you would be better off keeping something else.
the setup sounds like it should work, in that list, is there a skimmer? Keep the lights dim, and make sure not to bother the little guy too much, they die very easily from stress.
There is quite a debate on whether or not home hobbyists should keep deadly animals (or even potentially deadly !)...I think that it is fine, as long as the animal is kept away from the general public, ie: locked doors, etc. I maintain a small collection of rattlesnakes that are dangerous, hence a seperate room with a locked door, and I have the only key.
Working with blue rings and their relatives is really rather dull, though...they are very, very shy, and tend to not do really well in captivity.
It is your call, though.