Yeah, I was gonna say...how long was this going on? Rats aren't really very sensitive to newcomers in my experience(always exceptions, of course...but I've never come across them with fourteen rats). I was going to say to try it out for a day, making sure there was no blood drawn or anything...squeaking is okay. It's good you gave it some more time and let things iron themselves out. Animals are stronger than humans.
Hahah and if a baby was nipping one of my older ones I'd probably say too bad...the older one will just have to run away or discipline the younger one(considering it was truly a baby, and of course not letting it go on for toooo long).