This is a easy question for any of us to answer for you.
BUT BEFORE ANYTHING THANK YOU FOR ADOPTING FROM A SHELTER.
But before I answer this  PLEASE make sure you have all the vaccinations and
test for feline disease are done on both of them before you even try to
introduce them.   Specially have them checked for respirator aliments which
seem to follow shelter adoptee due to being in a multi-cat environments. I
work in a shelter it happens al the time plus some cat disease are spread by
a cat's sneeze.   If the male cat is not neutered yet  have it done before
any new kittens are produced with the new female cat.  PLEASE NO DECLAWING

    First this will take some time or not it might take as little as a day
or months.  You need to keep the new kitten in a separate room with a door
let them get to know each other from under the door.  Than after the hissing
and uneasiness' is over you will be able to tell by them playing with each
other under the door with no fighting involved.  You need to let her out by
herself to explore the house first with the other cat separated gives her
time to get over the new area shyness.  I can almost guarantee in the first
few minutes she will find the other cat.  Do this over a few days for short
periods. You can let them out together observe them to make sure no real
fighting happens ( you will know if they are really fighting and not play
boxing).  If any fighting happens separate them immediately no punishment
put separate them in the closed off rooms for a few more days.  After this
open the door and let them be go to a mutual room watch some TV have some
cat treats with you  let the furballs come to you.  You make sure you show
them both mutual attention no special favors or attention.  You will have to
be ready to become slaves to 2 furballs after this.

I am going to give you a little personal advice about having cats.  When
they are young like this you need to get them used to being handle.  You can
do this by taking a grooming brush and gently brushing them  while you are
brushing them you can start to examine their ears, teeth, touching their
paws and other areas.  You do this so the cat will be used to being handled
in case you need to have a vet examine them and so on.  Too many cat owners
forget that the furballs may need to be seen by some one else and we may
need to give them medication.  Phil's site has a great mini exam on it

If you have any other question we will all be glad to answer them.  Hope
this helps

http://www.fanciers.com/cat-faqs/getting-a-cat.shtml is a excellent site to
help out in this new challenge

this is Phil P's site  he is not a vet but one of the most knowledgeable cat
people I have meet http://www.maxshouse.com/Database_toc.htm

Matthew

I'm sure the introduction will go on fine as the new kitten is female as he
has no competion, anyway a special intro may not be needed at all.
keep them in separate rooms close to each other for a day or two then put
him in the room what she was in after that nothing should go wrong