True, but all the other points apply. Dart frogs and mudskippers have very different requirements and cannot live comfortably in the same enclosure. Whether mudskippers would attempt to eat dart frogs, I don't know. If the frogs were small enough, probably.
Mixing terrestrials can be done, and I think it makes for a great setup when done properly. BUT you should always keep the different species separately first, and become intimately familiar with their requirements and temperaments before even thinking of combining them in one cage.