To answer the original question it would probably be quite possible to house several different types of reed, glass, clown, or hourglass tree frogs with several different types of darts within a suitable enclosure if the keeper knows what he is doing and knows what to watch out for. It has been done before and will be done again. If you attempt such a thing though you need to know enough about frogs to know if the species your thinking about mixing are bold eaters(don't want one species eating all the food), share the same environmental needs, are about the same size, if they are likely to be aggressive, territorial or easily stressed etc.

That is just a incomplete list of factors to consider and there is quite a bit of potential for issues to arise if things are not carefully monitored. Consider it something for advanced keepers only. Even most advanced keepers don't mix very often because there is rarely a good reason to do so and it increases the chances of problems.