To treat anything that has chlorine and chloramine just get a tap water conditioner for aquariums. You can try any. I currently am using amquel since it says it takes out nitrates and nitrites as well. So just choose one I guess.
2. Fire belly toads are a good choice. Personally I think 5-6 could fit comfortably in a tank this size.
3)Other species you can keep in here BESIDES the fire belly toads are:
1)African clawed frogs(3 at most fit perfectly)
2)African dwarf frogs(5-9 since their tiny)
3)1 pacman frog
4)3toads(small sized 3-4inch)
5)Chorus frogs(2-3 at most)
6)Painted chubby frog(3-5)
7)American green tree frogs(they are rather small so 3-4 is okay just make sure you provide lots of branches for climbing. That SHOULD in MY opinion be enough for them)
Iam sure there are some mores but I cant think of anything else.

As for poison darts. Ived got no experience on them. But I see them at my friends house he has the blue ones with the black dots. He doesnt have much trouble with them. Like any other frog their not OVERLY hard to care for its just they need certain things. For example they need coverage, they need small foods, they need high humidity and high temps. I dont know the exact details but Iam pretty sure some people on here can give you the details. As for feeding them, you know how you feed a chameleon? You put the insects in a cup and let the chameleon eat it out of the cup. It MAY work for the frogs. You can feed it stuff like pin head crickets, weavils, and fruit flies. I dont know if they may take even pheonix worms. Pheonix worms are the size of half an inch. I mean I bought some myself and was rather disapointed at first. But I ended up using them anyways(they were too small for the animal I was going to feed em too). You can always get a high tiped dish. Make sure you put small quantities and feed them there. Voila.

Well this is all in my opinion. So I hope this helps.