Gotta say that that rule is not a good rule to go by. Some fish are massive waste producers, some fish require more room to swim. Some fish are aggressive. Also, if that rule was true, he could keep a 14 inch Oscar in such aquarium, since it's 7 USG. Would that be fair to the fish? No, so please, don't follow all the tips your LFS gives you. Some are insane but, some are great. Best to go over tips and rules like that on a forum.
Now, that that has been handled. Not trying to be mean BTW. But, you could throw in a black divider and put in 2 males Bettas. Or, you could add a school of 6 neons and a pair of Badis to. Note: Badis are hard to find in my experience.