I currently have 3 green tree frogs and 2 gray tree frogs in a 125 and have had them in there almost 2 years now,never had any issues with agression or death....theres more than enough room in the tank and they absolutely love it,ive heard some people do barkers with greens as well but ive never done that barkers get pretty big. All of my frogs are cb tho,if you catch a frog in the wild you run the risk of parasite or bacteria....but mixing diff species can also caue stress so your always risking your frogs life when mixing. There arent alot of frogs you can mix and most people will tell you not too...isoon will be adding clown tree frogs with my bird poop frogs, but there fine and the breeder i get them from said its perfectly fine to,same size,food and habitat requirments. IF you were going to mix grays and greens i would do a 55gallon no more than 7 frogs total with LOTS of greenery and hiding spots. You'll get diff opinions on mixing just dont go mixing waxy monkeys and red eyes,like i said theres very few species that mix successfully thats why i have 9 tanks currently lol