Most animals that utilize the same resources view each other as competition and therefore do not appreciate each other being around.
This does not mean it is impossible to mix such species, but it does create additional difficulties. Increasing "resources" and dividing territories with breaks in line of site will decrease the risks, though it will not remove the risks...
Be sure to learn how to watch for signs of stress as well as acts of aggression in the species you are working with. Many frogs have very subtle "acts of aggression" that are easy to overlook, though they still put stress on their 'victims'...
Thanks for the tip. I have a couple of greens and a gray. They don't seem to mind each other at all. I was thinking about getting a redeye to go with them, but considering it is tropical, relatively expensive and more fragile, I think I may just stick with what I have.