I always always always preach quarantine for a minimum of 2 months. This way any potential issues with the new frog cant get your old frog sick.

Unfortunately for the care I will have to let someone else chime in. Every single animal that I have I keep solitary, and will always preach solitary housing due to potential issues that may arise (apart from the few social species that are out there), but I am FAR from a frog expert.....