Join the crowd. My situation is a little bit different as my 11 year old, Mimi, has been the "only cat" for her entire life. Butter adopted us about six weeks ago and it has been turmoil in this house. Butter is approximately 4 years old (vet's best guess) and he is obviously a little more energetic than Mimi. I had him neutered immediately, but one hiss or growl from her and he is ready to play. They aren't physically fighting though, and she just tries to stay out of his way. I'm not sure they will ever be good friends, but they are making it together so far. Both of them have no problems with our two year old dogs.
Good luck. Just be patient.