I think you are missing my point Lady medusa, let me try to make my point clearer

Yes, it can probably be done without hurting the animals (stress, toxins etc.), if you have a very large enclosure, they are from the same region an so on. However, what I'm trying to say is that it's never actually good for the animals. It does not make them healthier or less stressed. I might not stress them, but it will not relieve stress.

If you make a list of what might happen, it would probably look like this:

Pros:
-At best, they will not harm each other

Cons:
-Might cause increased stress
-Poison each other over time (amphibians)
-Possible aggressiveness

Do you see what I'm saying? I'm not saying that it can't be done properly, I'm merely saying that even if you do it correctly, it will at best do nothing for the animals.