If you are going to add another cat you need to be able to keep them seperated and take introductions at their pace. You also have to be prepared for them to alternately love and hate each other and you until they are truly comfortable around each other. You also want to make sure you have and are adding curious, open, happy cats. If there is any stress to begin with it will be magnified times 100 with a new addition.
My last addition, my Bengal, was about two years ago. The other five were adults too. The Bengal was not out of her safe room unsupervised for about two months. I know that sounds extreme, and she did have some health issues we dealth with, but they all get along fine and I don't any behavioral issues with any of them.
In short, time, patience, and a plan B, should the cat poo hit the fan for longer than you can take!