It's definitely NOT wise to mix species under ANY circumstance. As Deac77 said above, just because a pet store does it does not make it OKAY to do.

A "White's Tree Frog" , "Dumpy Tree Frog" and "Australian White's Tree Frog" are one in the same frog, just different names...but all the same species, so there's no mixing there. However, the "Big Eyed Tree Frog" is a different species and I would suggest removing them from the White's Tree Frogs. The White's will get bigger then the Big Eyes and will more then likely eat them as they will be much smaller then them when fully grown.