There's a fomula to figure out your surface area and how many inches of fish you can stock. I don't remember how to do it, but maybe someone else that does can chime in. A hex tank doesn't offer much surface area. If I had to guess, I'd say 17 inches of fish. Also because the hex tank does not offer much horizontal space, I would stay away from streamlined swimmers, unless they're something that doesn't get big, like white clouds or neons.
Personally, I would put 1 angel in there, and find suitable tankmates for it. Angels don't swim fast, nor do they like to do "laps". I'd say they're an ideal canidate. I wouldn't put more than 1 angel, though, as they can be teritorial. With the limited space, you may encounter problems with more than one. JMO