Welcome to the forum. The majority of frog enthusiasts will tell you that mixing species is a very bad idea. I wholeheartedly agree with it. Aside from the fact that most amphibians in the pet trade are wild caught around the world and putting them together can pass their exotic disease between them, they also produce toxins that are toxic to other species, including other frogs and toads. The following article is written mainly about newts and salamanders, but it also covers fire bellied toads, and I believe it mentions tree frogs as well. All of the info applies to frogs:
Caudata Culture Articles - Species Mixing Disasters