Need help with girls fighting

Question
QUESTION: I just recently took home a female rat (Shana) almost 2 years old. She had lost her last cagemate and her owner asked me to try and introduce her in to my group consisting of 4 girls and 1 neutered male (all between 1-11/2 years of age). The initial gradual introductions went well and she lived peacefully with the others for 12 days. Suddenly she and one of my other girls (Grace) got in a fight i heard noise from the cage and separated them at once as I saw that there was blood. Both girls had about 3-6 smaller puncture wounds piercing the skin. None of the girls have a history of biting. Up till this point Grace had seemed a little apprehensive towards her new cagemate and the two had not interacted as much at the rest of the ratties.

I have tried to have them meet again outside the cage but both rats puff their fur when facing each other and seem rather hostile towards each other. I have tried to keep them occupied with treats in these short meetings and they do eat their snacks but as soon as they have time too sniff each other they seem start to aggress. Both girls get along with all other rats in the group.

I am very upset about this and have never experienced bitting in rats prior to this, and I am very discouraged by the situation. It is horrible to see my rats hurting each other in this way, I just want my rats to be happy and safe.
I have a backup option as Shanas former owner will be willing to take her back and then possibly borrow 2 younger rats from a 3. party as long as Shana is alive.

Is there any hope of the to laddies living together in peace? if so what can I do to make it work? or will it simply be too dangerous to try to make it work?
PS. Sorry about poor language; I live in Denmark and English is my second language.

ANSWER: Hi Maria

First of all, your English is impeccable! I would never have guessed it was your second language.You write better than many people whos one and only language is English!  LOL!

Now, about little old Shana.
I have studied behavior in rats so much that my husband said I am part rat and all I need is a tail!  What I do is think of how the problem rat feels and evaluate the situation.
Shana lost her cagemate, probably another girl she had known all her life (2 years) Next thing for Shana is to not only be without her only real friend but she is now without her keeper that she was familiar with and trusted. SHe was taken from her familiar surroundings and put into a new environment, not knowing anyone around her and not even familiar with the scents she smelled or the sounds she hears. Also, you did not say how many rats she lived with but I am going to assume it was just one, and she died. So now Shana has to deal with five strange rats on top of it all. I must say, I am surprised they lasted 12 days without any aggression showing from her or the others.
So lets break it down and see it from Shanas point of view:

1. Shana has to adjust to the loss of her beloved cagemate
2. Shana finds herself without her keeper, the only person she could trust to keep her safe, feed her and keep her clean and give her attention etc...
3. Shana finds herself in a new home with unfamiliar sounds, sights and smells.
4. Shana is unfamiliar with her new keeper and has to learn to trust that she will feed her and keep her safe.
5. Shana is now in a strange cage with strange rats she has never seen before.
6. Shana is used to living with only one rat, now there are five.
___________________________

So there you have it. Shana has ALOT to deal with and its really easy for people to forget to think of how the rat sees things. Rats really do experience all of the above: Stress from change is one of the most common cause of aggression rats, male or female.
Everything happened too fast for Shana,in my opinion. She needed more time to adjust to everything before going to live with strangers.
You can see if there is just one other rat in the mischief that she gets along with and put them together. Try each rat and switch them out to see who she seems to bond with the most. Save Grace for last, but be sure Grace knows that Shana is still part of the mischief and intends to stay that way.
I would suggest you not return Shana to her original owner unless she plans to keep her for good. This switching back and forth can really upset a rat that is ultra sensitive to change and Shana has had alot of that lately.
Give her more time as I stated above,and see if she wants to select her cage mate on her own accord. Grace may finally accept that Shana is here to stay and after a few weeks of still smelling each others scent, they will realize that they are part of each others mischief, like it or not.  If not, I would just let Shana live with one of the other rats or again, take turns with  them, with the exception of Grace.  
Hope this made sense.  Let me know what you think!

---------- FOLLOW-UP ----------

QUESTION: Thank you so much for Your very helpful reply.
I am afraid I was not clear about one thing though: When I separated the girls I took away Grace with a friend and. Shana has been kept in the main cage with three of my other rats. I did this mainly because of from what I saw Grace was the main aggressor, not Shana. Shana has seemed to respond to the dramatic change very well and appears to have bonded a lot with the other rats so I did not want to separate her from her new friends after the traumatic event. I am afraid that it is Grace who has responded poorly to the stress of having a new rat living with them. She has been a while adjusting to new rats before (it has been a while since I took a rat in though) before and has seemed a little dominant with newcomers but never drawing blood until now.

Does this affect how I should work with them and the prognosis?

Answer
I am so sorry it took so long to reach you.  I am on vacation and put in a request that I was going to leave for a few days to put my account on hold status but it didnt go through!  So I appologize.

I am also sorry for the confusion about the rats. DUH me!  It doesnt change the prognosis much, just put the shoes on the other paw, so to speak....think of the changes the other rat has to endure although not as much as the other rat that I first dicussed, but still she has to get used to the other rat being there and she may have felt like she had alpha role and all of a sudden, there is competition!

Hope this helps clear things up a bit!