introducing a baby rat to adult female

Question
I have an adult female rat (Katy), almost 9 months old, who
I got from my biology teacher.  I also had another female
(Sarah) who died last weekend, and they always got along
well and did almost everything together.  I know it's not
good for rats to be alone, so my biology teacher gave me
another young rat yesterday to be Katy's new "sister."  My
teacher does not know the age of my new rat (he simply
raises rats to feed his classroom snakes), but I estimate
she's about 4 weeks old.  Most of the time, they are ok, but
Katy pins down the little one a lot.  I've read this is
normal and fine when a new rat is introduced...but is it too
much for a new baby rat??  Last night I kept them in
separate cages, and I have put vanilla on them as I have let
them interact in the bathtub, my lap, and a neutral cage.  
They are fine until they go into Katy's cage. I have cleaned
and rearranged Katy's cage, but they still sometimes fight,
although it has gotten better than it was yesterday.  Is it
best for me to leave them alone and hope they get along, or
should I keep them separate (and alone) until the little one
is older? Do you have any other tips for me?  Thank you so
much for your time--I appreciate it so much!!!

Answer
The rule with rats is "no blood, no foul". Unless Katy is actually trying to hurt her little sister (and by hurt, I mean ripping out hair, screaming like a banshee, chasing each other around and causing wounds), leave them alone and let them work it out. Every time you separate them, it makes the whole cycle start back over again.


Good luck!