It's always hard to predict how rats will behave when a stranger is introduced to their territory. The bathtub (presumbably) is neutral ground, not claimed by any of the rats, so you don't see any territorial aggression there. Try this first: take your current rats out of their cage and put the baby in alone. Let her explore for an hour or so, until she's comfortable. Then take her out, put the resident rats back in. They'll smell the newcomer immediately. How do they behave? Do they get all puffed up, stomp their feet, rub their sides along the walls of the cage? These are territorial behaviors that usually indicate aggression. If they're just curious, things will go more quickly for you.
Try introducing your new rat to your other rats in a play area that the resident rats are used to using. Generally, they won't have as much aggression as they do in their cage (but not always). Remember that you want to slowly build up the tolerance of the resident rats to the newcomer. When they're not paying any more attention to the newcomer and she's been accepted to the group, you can try cage introductions.
You never want to just put a newcomer into a cage of rats - even the smallest and youngest of rats can get attacked and it's pretty tough to get in the cage to separate a fight. Before you introduce everyone to the cage together, clean the cage completely, scrubbing every wire down until there is no scent left of the resident rats. Same thing with any toys, hammocks, litter pans, igloos, water bottles, dishes or anything else in the cage. Then put new litter in, put the accessories back in new places and I even put the cage in a new location. That way, it seems like a new cage to everyone. Then, on a morning you'll be home all day and all night, take everyone out for a joint play time. When they've lost interest in playing, put them all back in the cage together. The hope is that they'll be so tuckered out (and in the morning, it's their sleepy time) they won't have the energy or the desire to fight. Keep a close eye on them, though, because sometimes it can take awhile for any fighting to start. If they curl up in the hammock together, you're doing great.
Here's an article with a good method of doing intros:
http://www.ratfanclub.org/newrat.html
