Honda, when we say not to species mix, that means pretty much everything. Where to begin... there are so many issues when it comes to species mixing. I suppose the first being that you risk giving one a disease, parasite, or other nasty from the other species.
Next issue is that there have been tons, and tons, and TONS of first hand acounts of species mixing disasters. Really, these are predatory animals, and if it looks like food, smells like food, moves like food, probably is food.
There also might be agressive from the different species. One may be larger than the other and feel like king of the tank, so to speak, and bully everything else in there.
Next thing to think about it natural environment. Unless you have had lots of experience, I'm sure you would be unaware of what species come from the same locality. By this I mean, unless you do hours of research, I find it hard to believe that a frog you want to put in with your salamander is from the same geological area. The other thing about geological area is that it determines what kind of appropriate housing is crucial for a specific animal. An example, if you own a newt that is fully aquatic all year round in their natural habitat, and you place it in a terrestrial tank... you are going to have some serious issues my friend.
I guess the last and final thing I want to mention is toxicology of these animals. Some salamanders have poison glands that can, and will be toxic to other species. Same goes for frogs.
Best advice, do not species mix!! Stick with what you have in one tank.
If you wish to add more, create a serperate tank.
