Introducing new rats

Question
Hi. In all the years I've kept rats, the one thing I've never had the confidence to do is to add rats of another litter to existing rats. Out of a group of 4 sisters I lost one in November & it is crossing my mind that someone else will go at some point. I don't like to think about it too much but I think when I get to 2 I would like to try to introduce a couple more, so that i don't end up with one on her own. However, I do wonder about adding younger rats to older rats, especially with health issues as i want to be fair to them.
They are currently a year and a half. One of them has sterile abscesses and will be having her second op soon. She has some mobility issues for which I've made accomodations, but she's not too ill with it. She seems to have got used to it & has a good quality of life. Another has the begininng of what could be benign tumors or the same thing. The 3rd is a tiny little thing with bags of energy but she does like to challenge the girl with abscesses, though she (with abscesses) has always been top dog and the little one goes through phases of constantly chasing and nipping her. She usually gets put in her place in the end but is very persistant and can wear her sister down, so i'm worried she might bully any new-comers. What do you think?
What would your advice be regarding ages of both sets of rats and also the idea of adding neutered males? (less chance of mammary tumors etc)
Thanks

Answer
Hey Lyndsay!
I've seen petstore rats just being put in a different cage with only a little nit-picking!  Most health concerns don't get spread around. Usually when adding older rats the older rat will take "alpha" while the younger rat might try to test the older one usually with females there's not much fighting.
http://ratguide.com/care/behavior/introducing_rats.php
This web-site will give you more information.
Your younger rat will be alpha when the others come in basicly automaticlly as she is the one more familiar with the cage and is older. For the males if you REALLY want them sure go ahead but if you want to stay with the same gender then you get get spayed females and they have less of a chance of any tumor.
Hope this helped
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Emma