The last time I brought in a new stray she lived in a bathroom for a while and they sniffed each other under the door. This was probably for a week. Then the newcomer was brought out for supervised periods of time - however long I could spend watching over everyone. I can't remember how long we went on like that - not too long - until everyone seemed to behave nicely. They really adjusted to each other pretty quickly. I had two neutered males and the new cat was a fully functional female. In short order she start going into heat and one of the boys attempted to go into action. I quickly made an appointment to get her spayed. She and that cat always got along better than the other one. They all got along fine but she'd curl up and sleep with him but not the other one.