hi,
i know how you feel, i resued a 9 yr old cat from rspca, we had her for about 5 months when we decided to get a friend for her, a 2 yr old tom, she did not like it at all and hissed every time he walked passed and sometimes paws would fly but they never actually would fight. we have had new cat about 3 months now and they are still not best friends but they have acceped eachother, although new cat never had a problem anyway. just give them time is my suggestion.
good luck