Not all cats are put off by Citrus and isn't it going to affect Emily's cats as well? If you want to try something old dried out tea bags sprayed with Deep Heat scattered around works. Cats hate the smell of Wintergreen.

The problem is that your cats being newcomers are interesting the locals who are the 'Top Cats' and are trying to assert their dominance.

We had a similar problem when we got Dusty, our Rescue cat. The locals were used to having 'free roaming rights' in our garden but Dusty wasn't having it and chased them all off. It was mostly howling contests but there were a couple of bundles and the locals are now very wary about invading Dusty's domaine and retreat respectfully if confronted. He tolerates next door's cat and allows him in to share his food, but that's as far as it goes. Its all a matter of Heirarchy