Well done for give an unwanted cat a home
There's lots of info on this link about introducing cats.
http://messybeast.com/first-impressions.htm

--
Alison
http://catinfolinks.mysite.wanadoo-members.co.uk/
http://doginfolinks.mysite.wanadoo-members.co.uk/

Keep cats separate for a while - then after 14 days (after they have
had some time to slowly adjust to scent of one another) do one or more
of the following:

1.  Make sure all cats in the home are FELV and FIV negative and
vaccinated, and neutered.  (glad you rescued one from the shelter - way
to go!)
2.  INtroduce them and allow them to interact under close supervision,
keep a spray bottle handy (water only) should a fight erupt.
3.  Expect hissing and fussing, if a physical altercation occurs -
separate for a while and keep trying.
4.  Give love and attention to both - particularly your original cat to
instill a sense of security.  Don't leave any dirty laundry or bags on
the floor - they are an invitation for the one cat to leave a message
of discontent.
5.  If it goes well, let them stay out a while together, then -
separate again at bedtime or if you are not there to supervise them.  I
would not leave them alone together for a while - until you are
ABSOLUTELY sure they are getting along well.

Here are some pointers to help a cat friendship develop and potentially
flourish:

1.  Feed them canned food at the same time, do this in separate dishes
but somewhat near one another.....over several days repeat the process,
gradually getting the dishes closer each time.... For most cats -
canned food is a treat - cats do not usually fight over food like dogs.
In some cases cats that hate one another will compromise long enough
to eat a treat at the same time.  **make the food some really delicious
stuff that they both go crazy over such as some tasty canned cat food.

They will learn to associate one another with something pleasant and
positive.

2.  Get a feather toy on a long stick and initiate (and remain involved
in - through interactive play) a play session.  Sometimes cats will
become so jubilant about playing they forget about the fact that a
"new" cat is around.  If cats start interacting positively - as in
playing - then gradually step back and see if they will start playing.
If playing is too stimulating then - stop short - and play with each
kitty individually while the other watches....

3.  Cat nip - It sometimes lightens the cats mood and makes them happy
and playful.  If consumed it makes them relaxed, often cats like to
injest a little of it.  Use sparingly.

4.  Add a few additional litterboxes to your home so that if any
territorial issues emerge there are plenty of CORRECT places for cats
to go.  Use a brand of litter called cat attract until kitties start
getting along.

5.  Buy a kitty card board scratcher as an outlet for frustration and
release of energy and
appropriate clawing which is good for cats.  this will release tension
for the cat.

GIVE CATS time - and make sure one is not bullying the other.  Good
luck. Jen

PS:   Occassionally I have crated cats (in a super large dog crate) and
brought them into the home where my other cats get a chance to slowly
evaluate the new cat - the new cat is not really invading their space
and they can come up and sniff the new cat without being threatened.  I
leave the new cat in crate for quite a few days with litter box and
food - this has also worked well - but is an alternative method of cat
introductiion.  The new cat does not get run of the house right away
and former cats do not have their territory threatened or compromised.

Cheers,