Just an answer to a couple of your questions. I have not tried the large millipedes with darts, but there is a smaller type that occurs as a contaminant, and these don't appear to bother the darts at all. I rather like having them, as they feed on the organic matter and help recycle the frog wastes. The larger millipedes MIGHT invade the frogs' territory, which would annoy and stress them. I'd try it on a small scale first--something you could quickly remove at the first sign of a problem. The mantis would be very unlikely to bother the frogs, but I don't know whether a tropical pet species would be capable of eating one if it bumbled into it. Lot's of "don't knows" here.
As for fish, I do have some mixing experience here, and it's been quite positive. I have two paludariums that are a combination of aquarium/terrarium. The aquariums sections are relatively shallow, i.e. 10 inches down to 8 inches and heavily planted. The most successful fish have been the various tetras, such as X-ray, rummy nose, head and tail light, etc., some of which have lived for several years. (I can't seem to ever keep neons for long, however.) Various algae eaters do fairly well, and I have one Yoyo loach for snail control that has been around a long time. The water sections circulate over water falls, small "rapids" and in one tank, over a tree fern fiber drip wall. They have been low maintenance compared to a regular fish tank. In the larger tank, the water is circulated by an external canister filter, and in the smaller one, (135 gallons) with just an internal 600 Rio pump returning the water over the falls. All filtering in this tank is done by living Java moss in the narrow "rapids" fine filament algaes, and colonization of the tubing and surfaces of the water falls by biofilm of nitrifying bacteria. In nine years, I have never done a complete cleaning or water change, although there are frequent partial changes and replacement from some leakage via the water falls. It actually does work extremely well over all.
You should have an easy egress to land such as a submerged log, water to land for frogs that may fall in by accident. While they are quite capable of swimming, they do a kind of dash to the nearest surface, and really aren't very good at it. In a relatively shallow water ways, i.e. the 10 inches as mine are, many of the aquatic plants will have leaves floating on the surface, and some frogs will take advantage of these to hop around on. (My D. galactonotus are in the water on top of the leaves frequently.)
As an aside, someone stated that dart frogs aren't arboreal. While there are decidedly arboreal species, even the so-called terrestrial species may climb several feet if given the opportunity. So there's no guarantee a snake/dart frog path will not cross, even as high as 8 feet upward.
All in all, your project sounds like it will be really fun to plan and do, even if you have to alter some of your mixing ideas as you learn more along the way.
One of the first things you want to do is get rich!
Here's a couple of photos of the galactonotus playing in the "river run" part of the aquarium, and also climbing the glass after a "swim."
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Patty
Pahsimeroi, Idaho
D. auratus blue, auratus Ancon Hill, galactonotus orange, galactonotus yellow, fantasticus, reticulatus, imitator, castaneoticus, azureus, pumilio Bastimentos. P. lugubris, vittatus, terribilis mint green, terribilis orange.
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