I wouldn't try to keep a dart in anything less than 10 gallons, Even though the general rule is 5 gallons per frog, the minimum size should be 10. They are generally pretty active, and the size of the frog isn't a good criteria for space either, since some of the very tiny thumbnail fellows really get around.
If you can squeeze in a 10 gallon, you could keep a couple of Dendrobates leucomelas, auratus, or imitators (same species of course, not mixed)as these get along together regardless of sex, or perhaps a single tinctorius, or azureus, that are more territorial and are best kept singly or in mated pairs. There are other good group frogs among the Epipedobates, but I have no experience with these. Other nice choices for two unsexed frogs would be Phyllobates terribilis or vittatus. P. lugubris is a really tiny little guy that looks somewhat like a miniature P. vittatus, but these are hard to find. D. galactonotus are my very favorite frogs of any, but they remain rather expensive and from my experience, do best with a lot of space in groups of 6 or more.
You can also get recommendations from whatever breeder you choose, as this advice is mostly my own opinion based on personal experience. Other people may have had different experiences.
You will need to culture some flightless fruit flies, and that does take some space. Although I've recently read an article about them needing a light cycle, I've cultured them in a closed cupboard quite successfully. You'd probably need to keep 4- 1 qt. cultures cycling every week over a period of a month.
If you can manage the space for this, darts are certainly delightful frogs to keep, and a planted vivarium recycles wastes and needs no substrate changing, with minimal upkeep.
Here's a couple of photos of a 10 gallon high hex with water fall under construction.



-----
Patty
Pahsimeroi, Idaho
D. auratus blue
D. galactonotus pumpkin orange splash back
D. imitator
D. leucomelas
D. pumilio Bastimentos
D. fantasticus
P. terribilis mint and organe
D. reticulatus
D. castaneoticus
D. azureus
P vittatus
P. lugubris