Mixing Fire Bellies with anything else is pretty frowned upon, they are toxic. So that water is going to slowly and slowly get more and more toxic, and this will eventually kill the fish or make them sick. Fire Bellies live very well together though, you could have at least 2 in a 7.5, maybe three.

Tree frogs seem to tolerate being handled more than other species I've kept, I dont know about green tree frogs in particular, but as everyone said it's not a great idea.