Introductions need to be done slowly, especially with cats who are no longer kittens.
You should keep them separated for a couple of weeks, periodically exchanging blankets, pillows, beds so they can get used to each others scents. If you are able to let them smell and touch each other through a door this helps as well.
I would not have put them together until the new cat got a check up from the vet.
I took in a stray last year. She's about 5 now, and I didn't bring her into my home until she was checked out first. After she was examined and found to be healthy, I brought her home and had to keep her in my kitchen, away from the others for several months. My kitchen doesn't have a door so I had a screen. My girl pretty much stayed inside her little bed for a month, until she began to feel comfortable enough to venture out.
It is very stressful for a cat who has lived for so long outside, to come into a home, especially with another cat.
You have to think, you have taken them out of their universe, and stuck them in a totally different place. It takes time for them to get used to things and realize no harm will come to them.
Here are a few sites you might want to look at:
http://www.our-happy-cat.com/introducing-cats.html
http://www.fourpaws.org/pages/adopti...cing_cats.html
Here's a video:
http://www.expertvillage.com/video/1...g-new-cats.htm
Good luck.