Darts frogs come in just about every color yuou want. The two you just posted are pics of azureus ( the blue ones) and leucomelas ( the orange and black) which should never NEVER be mixed together like that.
I have a couple of costan ricans I will try to post some pics of them ( as soon as I figure out how to do it.
Dart frogs in the wild get to be poisionous from eating a variety of different things not just "poison ants" All captive bred dart frogs have no toxins on/in them what so ever. Even wild caught darts loose their ability to porduce toxins after living in captivity.
The tank does not really cycle like a fish tank does. It is best to wait a few week before getting frogs so you have time to find out what plants are going to live and which are not and then you also have time to make sure your temps and humidity are where they are suppost to be.
Another good thing to do after you get your frogs is QT them for about a month. Dont put them directly into the tank. A steralite container works well for this. You need to make sure they are eating and healthy before they go into the main tank. Good idea to get a fecal done too. Shame to put all that work into a tank to find out your frogs have lungworm hookworm or some other parasite just to have to tear down the tank.
There are many different types of grren and black auratus. ( costa rican, Hawaiian, Panama, ancon hill) make sure the person you get them from know what morph they are and DO NOT mix them.
As for the fungus part. Almost anything he collected from the outside of his house will mold. Alot of things you put into the tank will mold but it breaks down after a decent amount of time ( white fuzzy mold is harmless) springtails will also help keep the mold in check. Some of my tanks have been set up for over 3 years and they still sprout a mushrooms or some funky type of mold every now and then