I think Skronkykong answered most of your questions, but I'll add my two bits, now worth about a penny. he darts you mentioned will do well alone in a 10 gallon. Just have a drainage layer of gravel in it, the coco-fiber/sphagnum or organic compost over this, separated by a layer of weed cloth. Excess water can be drained from the gravel layer by merely inserting an airline tube in one corner to the bottom and siphoning it off whenever it accumulates. Plant some small, live tropical plants and add some leaf litter such as oak or magnolia leaves. I use a hinged glass top to hold the humidity, but you can also put Saran Wrap over a screen if you prefer. You could keep two (unsexed) auratus in a 10 gallon, or a single tinctorius. The auratus get along well together regardless of sex, but the tinctorius, especially the females will fight unless kept in mated pairs, and you would need a larger tank, such as a 20 gallon to keep them happy, as these like more ground space. You could also consider leucomelas, one or two, which also get along regardless of sex. Another nice group friendly dart frog is Phyllobates terribilis, although a 10 gallon seems a bit small for them. Live plants will recycle the wastes adequately. I particularly like to add Ficus pumila "quercifolia" as it forms a very nice, low, tight groundcover, will climb backgrounds, branches and other wood features, as well. Most pillow and sheets mosses have never thrived for me, but Java moss, although longer, does. All darts appreciate a cave or hut of some sort, such as a coco-hut.
As far as fruit fly media, although I usually get mine commercially, in a pinch I've used dried potato flakes with a bit of corn meal, topped off with about 1/4 tsp. of Baker's yeast after adding the water, and this seems to work just fine. You can also add a teaspoon of vinegar to the water when diluting the media to inhibit unwanted molds, if these are a problem. Adding some excelsior to the deli-cup increases the space for the flies and seems to increase production. Excelsior is simply the shredded aspen bark you can find in crafts stores.
Of course, you need to dust the fruit flies with a combination of something like Rep-Cal with D3 and Herptivite, or use Dendrocare. The D3 is important, since in a small tank especially, there is no convenient way to provide UV light, and D3 is needed for the frogs to utilize the calcium.
Most reliable breeders ship their frogs as juveniles rather than as young froglets, and these do as well as adults in my experience. Sexed adults will be more expensive.
I think dart frog breeders are a different "breed" themselves, as I've found those I've dealt with to be strictly honest, and won't tell you a particular species is "easy" if it's not. They also provide personal advice, care-sheets and healthy live delivery guarantees. After that, since they have no control over your conditions, you are on your own.
Here is a 10 gallon I set up three years ago for two Phyllobates lugubris with a small circulating water way. It is currently still running without ever having been taken down and "cleaned" except for cleaning off glass and trimming back plants.
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Patty
Pahsimeroi, Idaho
Dendrobates: auratus blue, auratus Ancon Hill, tinctorius azureus, leucomelas. Phyllobates: vittatus, terribilis, lugubris. Epipedobates: anthonyi tricolor pasaje. Ranitomeya fantastica, imitator, reticulata. Adelphobates castaneoticus, galactonotus. Oophagia pumilio Bastimentos. (updated systematic nomenclature)
Danne