I'm not sure what the rule is or if there even is one, I posted my 10 gallon Dennerle tanks under nano tanks, and this is 15% the size. These Mr Aqua rimless tanks are very neat designs. Curious to see how you'll like maintaining this one with such a narrow space between front and back glass. I guess if a hand can get in there easily it should be fine, but hardscape would get very interesting. You seem to have found the perfect piece of spiderwood for it.
Nothing good happens fast in an ecosystem.