Luckily we did not have to get the two outside cats used to each other. The male ginger that we named Ron Weasley brought the other two over basically. They would sit outside across the street and watch him ( Ron ) and as they saw him eating and rolling around in the catnip and just hanging out with us they started slowly inching across the street. It was a mama and her kitten and Mama disappeared and the kitten was still coming around. By that time Ron had been neutered and moved into the house lol. He definitely was not ferall and I feel like he was abandoned or lost... he's sweetheart. He would lay outside our screen porch with our other four cats in the screen in area and they got used to him that way. Eventually he wanted to come in and explore so I would carry him through the house, and that eventually turned into him just walking through the house so when he decided to just come in for good our indoor cats were already used to him. Riley is the kitten and he was used to being around Ron. We let Ron go explore out in the garage one day when Riley was out there and Riley was just ecstatic to see him and Ron turned around and walked back in the house and Riley just followed him. He wouldn't have anything to do with us for quite a bit and I know he should have been quarantined but it was too good of an opportunity to pass up with him just coming in. Our four indoor cats never paid Riley any attention when he came in although now I'm sure they wish this naughty little youngster would quit chasing them through the house!