Hoping there are some multi-cat people here that can help??
I posted a few weeks ago about taking in our ex-neighbours cat, that was abandoned after they moved away. He didnt settle where they moved so wanted to take him to a shelter!!! We refused to let that happen and took him in as a trial, to see if he would settle with our two cats. While he seems very happy here, our other cats are not at all, particularly our girl. She is either growling and swearing at him, or hiding upstairs. She's also taking to staying away from the house a lot.
I think the main issue is they were all previously territorial in the neighbourhood & we've now let the 'enemy' cat in. DH thinks the new cat might be better off with the shelter, but I think it needs more time. I'm at a loss though as I think we've done absolutely everything to help (slow introductions, feliway, safe places, more food bowls etc), and a month down the line things aren't any better.