I successfully added a baby to a 2 week-old group. I did it the exact same way you added the male in your above post. I put the baby in soiled bedding, then I cleaned the enclosure. I put the baby in with the soiled bedding and piled the rest of the babies on top of her. Then I put the mom back. She is now a breeder in that same group!

I've also made the mistake of returning a 3-week-old rat that wasn't eaten back into the colony. I felt terrible because I've read on here about them not very good with "other" rats, but I only had the rat out for less than an hour so I didn't think they'd forget about him already! I woke up the next morning and the rat was dead and buried in a corner.