I believe I will take Mr. Yeager up on his invite to continue this discussion.
When referring to mixed enclosuresi the use of the word mixed is an inapt description as this indicates that the animals are together in a homogenized fashion. Because this is not what typically happens multispecies is a better description of the properly set-up enclosure.
Multispecies enclosures are becoming more and more common with the larger and better Zoos (including some of those at the forefront of dendrobatid breeding such as NAIB) and Aquaria. Many of the multispecies enclosures at these institutions have been present for years (some for more than a decade) with little to no problems and in some occasions house multiplegenerations of the animals on exhibit.
There are a lot of issues that are thrown when ever multispecies enclosures are brought up on various forums (not just this one) these include (and I am sure I missed a few),
1) the spatial requirements of the animals are violated
Spatial needs of the animals, this is a issue where some hard and fast numbers have become set in stone in the hobby. Usually, people speak about 5 gallons per frog. To make this simple, I am using the assumption that the 5.5 gallon tank is the standard for the 5 gallons that is the commonly used reference.
Within the 5.5 gallons of space, the space used by the frog (I am going to use a tinct as a standard for the larger dart frogs) is typically very different than the space "alloted". In a typically planted set-up the frog will only use the bottom of the tank most of the time so the actual used total space can be calculated by the surface area of the bottom of the tank (8 inches by 12 inches) and say 3 inches of head room for the frogs to hop. A 5.5 gallon tank contains 960 cubic inches so the frogs only really use 30% (288/960 = 0.3) of the available space or about 1.65 gallons.
What this means is that people have to pay attention to how the tank is portioned out for the frogs. With the set-up described above (5.5 gallon tank) there may be between 4 to 5 inches (substracting for bottom of the tank) of height in the tank that is not used by the dart frog. This would indicate that another species could inhabit that niche if the owner of the enclosure was so inclined. In a manner similar to aquariums, people can look at the enclosure as having an upper portions, a middle portion and a lower portion. In most of the smaller enclosures, this will only be two levels. The other space designates a niche that can potentially be used for a different species. This is where multispecies enclosure planning begins. The person needs to be aware of how much space is really being utilized in the enclosure by the animals. This will give you the first step on the path to the next choice if you wish to keep multispecies enclosures.
After comments (if any), next topic pathogens (last will be criteria to help make the correct choice).