Difficulties with Rat Intros??

I've been working on introducing two baby girls to my two older girls.

I've been cage swapping them, and the rats do not show any signs of aggression in neutral territory. I've attempted to move onto play in territory familiar to the resident rats. One of my rats seems to handle it quite well, but the other one will get all puffed up and then bite one of the little ones. There has been no blood thus far, although I'm wondering if I'm intervening too soon.

The one that is showing aggression now is not actually my alpha of the pair (to the best of my knowledge). She is my timid girl though, she's always been quite anxious.

Is that amount of aggression acceptable? Or am I right to intervene?

I've continued with the cage swapping, and once there's aggression in the non-neutral area, I put them all back into the neutral area.

Suggestions would be appreciated!

Intervening soon is good, especially when you are dealing with an adult and a baby.
How long into the intro does she get puffy?
How old are the babies?
You need to do a lot more intros in the neutral territory.

Here is some info that may be helpful
http://www.ratshackforum.com/forum/showthread.php?t=32368
and http://www.joinrats.com/Intros

good luck

If I go 20-30 minutes in the neutral zone, and then put them in the familiar zone, it may take her about 10 minutes before aggression.

This morning, I tried bringing out just one little one, and the aggressive one, and her aggression was basically instant. I intervened, calmed her down, and tried putting her down again. Same thing, some more aggression. She gets all poofed up and then bites them. No blood has been drawn at this point.

I find that in the neutral territory they are all too freaked out to even really scope each other out. They just kind of try to find a way to escape.

Thanks! I'll keep at it.