I have had cats all my life and whenever I added a new one to the group, I did not keep them seprate from each other. I just introduced them, played with them all for a while and left the room. Some cats become good friends some do not and will slap the one they do not like whenever they walk by but usually is not a hurting slap, just one to say "hey you are on my terf and I do not like you". Since you have kept them seperate I am not sure what you should do at this point. Make them spend time with each other for a few minutes in the kitchen? put all food bowls in same room. I have several outdoor cats that I have given them homes because they had to leave their former homes for some reason or other. I would put the new cat in a large cage like a ferret cage for a week so it could get used to other cats and the new home. It has worked out very well. The cats have a couple "cat houses" they can go into and the barn. It is kind of fun to watch them interact with each other and how friendly some are to others and how some are standoffish. Good luck. Take care
Linda