I want another frog!

[UncleChester]

They should be the same size- anything that moves and a gray treefrog thinks will fit in its mouth is something that it will try to eat.

In my experience, Grays are not in any way social. Not that they are anti-social either though, my two adults pretty much ignore one another all the time (the exception is Waldos futile attempts to woo the uninterested Fatty). Similar experience with wild ones outside of breeding time, they will sit on each other, but they also sit on rocks.

Male Gray's sound awesome, I love it when Waldo is trilling. They will call at night too which might be disturbing if they have to be close to your bedroom, so if that's a problem go for a female I suppose.

I would also discourage mixing species unless you are an experienced keeper. But don't let this stop you from getting another species- you can always make room for more tanks.