Just take the introductions very slowly. Every cat is different but hissing and growling can be expected even in the nicest cats. I've always found that introducing kittens to adult cats is much easier than introducing 2 adults. I think that my easiest adult introduction was Sky. I didn't have a roommate at the time so he was able to stay in the second bedroom for 1 week. I would slowly let him out under supervision and he got along with everyone and for the most part everyone also got along with him.

I've also found that it's harder for females to accept newbies than males. I've now had Alani & Blaze for almost 6 months now and both of my females Ziggy & Pearl will still sometimes hiss and growl at them. My males Storm & Sky are very good with them and they will even lick their heads. To me females seem much more territorial. My Pearl is also a Lynx Point Siamese mix and even though she's petite, she's very bossy, feisty, and demanding. She desperately wants to be the alpha cat but Storm isn't willing to let her have this position.

I hope that everything will go smoothly for you and your current animals. Good luck.