Well it is a good thing that you are upgrading, those small Exos are only about 5 gal! They need minimum 10gal for one and plus 5 each additional, minimum. So that is good.

You don't want to put anything else in that tank. It is a bad idea all around. You could get another gray, but really your two will be quite happy not being cramped with a bunch of others bothering them. Besides it leaves you room to put in some really nice plants and wood and vines.

Here is a thread on another site that may help with not mixing.
http://www.repticzone.com/forums/TreeFrogs/messages/806159.html