Nice. Definitely get rid of the biter. As you breed, hold back males and females that express the qualities you want. I have one hold back now that I got attached to a bit but he is skittish and nibbles my fingers too much/too hard, so he won't be getting bred and I'll probably find him a home on craigslist soon. It's amazing how you can see the personalities of the parents in the babies. I have one holdback that is such a little angel just like his papa. He interacts with me, new rats, and new experiences just the same as his dad - which is very passive and tame. He isn't shy but he isn't overly outgoing or overly confident. I'd put any of my rats in with them and not even think twice.