will 1 pygmy octopus and nothing else be ok in a 5 gallon hex tank?

Answer 1

http://www.advancedaquarist.com/issues/july2003/invert.htm
http://www.reefs.org/library/article/johnston_lanett.html
http://www.thecephalopodpage.org/sources.php

Problem is, that they have very short life spans and are capable of escaping through ANY hole.
And oh yes, they are poisonous.