I have two cats. 4 years of togetherness and they still hiss and fight. But, that's tough. I love them both and the house is big enough to share.
Funny thing, though. The younger cat HATES dogs. If the dog is smaller than a midsize dog, he will ATTACK any dog he sees.
My M.I.L. Brought her new puppy to our house and they were sitting on the couch with this puppy on their laps. My cat wanders in, sees the puppy and up he goes into a furry arch, hisses and literallly launches himself so fast up onto the couch to attch and kill that puppy. Only my FIL's quick defenses saved the puppy from a good clawing. He slapped my cat off the couch, but the crazy feline recoverred his feet and jumped right back up to renew his attack. It was unbelieveable!