It's great that you're getting a well-started yearling, btw. That helps. I'd personally be worried about the new ATB being infected by something the crested gecko left behind- remember that animals (or humans) can harbor & share germs/diseases without themselves being sick. I'd favor a full clean-out (disinfection), but that's me. Nature isn't that clean either, but when animals are transferred to a new home, the stress can lower their immune system functioning. I try to err on the side of caution, since herps are easier to keep healthy than to make them well again.