Behaviourists hear frequently from owners of multiple cats concerned that their pets are not getting along as famously as they had hoped. In some cases the relationship between cats is simply chilly. They ignore one another and perhaps occasionally emit a hiss. In other cases, owners are distressed by sporadic spats, in which cats yowl and roll together, then separate.
For the most part such cats live in relative peace except for the occasional squabble. Mismatched personalities (for example, a timid, middle-aged cat sharing a home with a young, rambunctious newcomer) are almost certain to result in conflict now and then. But rest easy: research has shown that argumentative cats will live together less contentiously over time, i.e., the longer cats live together, the better they tend to get along.
So if your cats eat, sleep and groom comfortably in the same room, even if they sometimes hiss at, chase or wrestle each other, they will probably continue to live well together over the years. On the other hand, if fighting suddenly becomes severe, it may be time to seek the help of a behaviourist.