Most if not all herpetologist will advise not to mix species as it will most likely cause stress to you animals due to many factors including but not limiting to : temp, humidity, size .......... and so on.
But if you do wish not to heed the warning as many keepers do not when they have a huge Viv....or tanks is my experience as a marine reef advisor .... keep to room-mates from the same habitat so require the same temp and humidity, get room-mates that are around the same size and will grow to the same size in adulthood and as your treefrogs are arboreal maybe terrestrial frogs would be a safer option.
My advice would be....get a few more green tree frogs of the same size and try breed them