I always recommend a 30 day quarantine to prevent any illnesses coming in with new pets. It's good to observe them, their health, eating habits, and feces for awhile before introducing them into the vivarium and with your frog. You'd be surprised at what they can bring in.
After rescuing several frogs, I'd like to recommend also getting a fecal test done on both your frog and any new frog(s). That way if they have any parasites you won't be contaminating each other or your vivarium. Most parasites can be treated with Panacur, prescribed by a vet.