I fell in love with a attention (and food) starved little stray. She was about six months old when I picked her up off the streets. I already have a six year old cat, and I took the introduction really slowly. Unfortunately, after only having the cat two weeks I had to leave the country for three weeks. The new cat stayed with my friend and acted aggressively towards her two cats (attacking them). Then my friend ended up staying at my place for two weeks. She was supposed to keep the cats separate, but she accidentally let the new cat chase the older one. The older one fell off the high cabinet trying to escape and is still terrified of the new cat.
I've been keeping them in separate rooms unless we put the new cat on a leash and my husband holds her while I sit next to my first cat. New kitty will eventually try to charge original kitty and then we remove new kitty. The new kitty is extremely people friendly, but I'm despairing of ever being able to have my kitties in the same room without constant supervision. We've been back from our trip for three weeks, and putting them in the same room with new kitty leashed for one week.