Albino rat nipping, hiding

Question
Hi. I have had an albino female rat, Ava, I'm not sure how old she is but I am assuming she is about three or four months old, as she is medium sized. I've had her for about a week and she is beginning to concern me with the frequency she nips at me. Whenever I put my finger or knuckle out to let her smell me before I pet her, she nips me very lightly, not painful but I can feel her teeth. I understand that albino rats are nearly blind so I am not concerned with her nipping, I just don't want it to progress to biting, and I don't want her to be nipping me because she is afraid of me. She also hides, alot - when I bring her out for playtime she just hides in whatever she can crawl under and doesn't really come out. She seems happy when she isn't in the open though - she allows my boyfriend and I to stroke her and scratch behind her ears, and she'll start bruxing happily. However, she doesn't like to be touched in the open at all, and if we do, she runs and hides. I'm concerned because I know they brux when stressed too, and I don't want her to be shaking in fear when I assume she's content. She also hides in her cage, runs back into her little log whenever I put her back. Her cagemate, Zero, is much more adventurous and enjoys running all around her play area, and also in her cage. I have been trying to give her time to adjust but the nipping concerns me. I am also considering getting another female rat (I have a large cage) to play with Zero just because Ava is so shy and doesn't want to play ... Would this make Ava more comfortable as well? Or would it only worsen things? Any help is very appreciated.

Answer
I always prefer three rats to two because if one passes away, there are at least the two left for each other.  Sad to think, but have learned over the years three isnt a crowd when it comes to rats.

Also it looks as if Ava was not as socialized as Zero was.  I believe Ava is only nipping to figure out who you are but means no harm. She needs time to learn to trust you and this will come as she lives there longer and gets used to you.  A week just is not long enough for a rat that has never been properly introduced to humans, which should be done at birth.  Give her time....she will loosen up.