The sooner you get them together, the better. But you need to take the needs of they youngster in consideration. It's really too bad you couldn't get 2, as it would be easier to indroduce 2 than to introduce 1. Any option for picking up an other? Is your brooder set up to provide the heat needed by the newbie? An excellent way to meet needs of all would be a heating pad. You might want to read Blooie's excellent thread on the subject. Keep a wire divider between the babies for the first day or two, and then, you can introduce them together for short periods when you can supervise. Don't leave them alone together, until you are assured that the baby won't get trampled or bullied. Again, IMO, a heating pad brooder will cut way down on that likelihood.