Megan, Nelly may never be best friends with your boyfriend's cats, but
they will grow to tolerate each other. I've taken in many strays, and
have *never* had one that did not eventually integrate.
The thing is, this has to be a gradual process, and you've got to keep
her isolated from the other cats. I can't say how long it will take;
every cat is different. The briefest time I've kept one isolated was
three weeks. But don't be surprised if it takes a month or more.
Keep her in an extra bedroom, if at all possible. You'll have to take
time out to spend with her. AFter a while, rotate cats, and put your
boyfriend's cats in the bedroom, and let Nellie have the run of the
house for the evening.
Ideally, the cats should be able to smell each other; I put a latch on
the door so that it would open a couple of inches. They could touch
noses, paw at each other, but were still separated.
Just don't expect miracles overnight. But don't get discouraged,
either; I bet in a few weeks she'll be much better and used to her
surroundings. Good luck.

Sherry.

I've taken in some cats on emergency basis, sometimes they
get along fine pretty quick, sometimes they never get along.  
Depends on the cat.  

A number of years ago I took in this tiny female kitttten and
my oldest female at the time never really accepted the kitten.
This went on for many months until I finallly found a horme for
the kitten.  

Cats are territorial creatures, they claim you and your house
as their territory.   And as I found a female cat may never
get to the point where she accepts another female cat.  

I've never found the secret to getting the current cats
to accept a new cat.   World War 3 starts and
in some cases never ends.

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