I agree with treybabe.
I have an almost 8 yr. old, and brought home a kitten last April. It took a bit of time for them to 'work it out' all the way.
The other thing was that they played sometimes kinda rough looking, but that's all it was- just cat play.
Are you sure it's not just play?
My cats are the best of buddies, but they get pissy with each other frequently. It lasts a few minutes, then they are all lovey again.