Ida, I'm sorry Penny's having a tough time right now.

Some time ago I had a bonded pair of male cats. When they were about 6 years old, we adopted a female stray. Everyone got along fine, but one of the males (Luther) started spraying inside, e.g., corners of the furniture, my husband's briefcase, you get the picture. We think it was a stress/territorial thing. I got Feliway spray and went to town. I had to keep it up for weeks and weeks, but it finally did the trick. I tried the plug-in kind and don't think it was effective at all.

Anyway, I don't think it can hurt to try some Feliway. Good luck with your girls.

I don't have advice, best years' sounded nice, but .here is VEB's (KingdomMirror) post about her foster cat and the kitten horde.

I have a Feliway diffuser in every room. My house is a battle ground without it. IMO it works. Be sure to get the multi-cat version rather than the one for inappropriate scratching.

I needed to integrate a cat with my dad's cantankerous and antisocial old male. I stretched their gradual exposure to one another out over a whole month. Feeding on opposite sides of a door works well. I think the most important thing is to not rush it. Miss Jenny had a bed, cat tree and litter box in a largish bathroom with frequent supervised visits out in the house. I put Cranky Old Man in her bathroom to get used to her smell while she had outings. I probably could have done it faster but I didn't have any other alternative for Miss Jenny if integration failed.

Bestyear's advice sounds right on.

Great photo of Penny! She's a Russian Blue, right?

Prozac, compounded into a chicken flavored soft pill, has kept our anxiety ridden kitty feeling OK. It's a long term solution for us (after many, many failed short term attempts). I'm not sure if it's something you can do intermittently but it may be worth looking into if she continues to be stressed by new things.

Good luck. I know how stressful it can be when your pet is not feeling well

I'd give the Feliway another try. It may not have worked "before" because Penny may not have been stressed at that time. Try it again, putting a few diffusers throughout the house. It's hard to drop a new cat into an established colony, no matter how small. Cats are pretty change averse and poor Penny's been through quite a bit..

Also try to make sure she's getting adequate water.

Thank you all so much for the caring suggestions, all of which make a lot of sense to me. In all fairness to Feliway, we didn't have Penny when we last used it, so we won't know how she'll take to it unless and until we try it with her. I'm thinking we'll pick up some of the spray, and I suppose it won't hurt anything to spray it all around the room where the litter box she uses most often is located?

It only takes a little of the spray. I would use a little spritz on evenly spaced surfaces throughout the house (door jam, carpet, bed ruffle, etc) so that she is exposed to it wherever she is, especially where she encounters Posy.

You might also try Bach’s Rescue Remedy. I was always skeptical but it’s very highly recommended by holistic vets and some devoted “cat people”. I used it for my oldie that has inflammatory bowel disease and anxiety when he started having litter box issues and I have to admit that it helped. He hates for me to force something into his mouth, so I would put the drops on a treat. It says you can rub it into their paw pads, ear or belly, too.

Bach’s Rescue Remedy, Amazon

If she gets canned food, you can add some water to her servings (gradually, a teaspoon at a time so she doesn’t reject it) to help with hydration and dilute her urine a bit so it isn’t as painful.

This water fountain has helped with hydration for my cats.

Ceramic water fountain, Amazon

That water fountain is interesting! Penny, especially, is fascinated with water and is the only cat I've ever had who doesn't mind getting a little wet. She's at her cuddliest when I'm fresh out of the shower and still dripping! I wonder if she'd like the fountain ... or if she'd just play in it!

Mine do both. I have it on a water hog mat. They’re definitely fascinated by it and drink from the water that trickles off the sides. There are tons of different sizes and shapes. This one is a good size for me. Go with ceramic or stainless rather than plastic, which can harbor cooties even after thorough cleaning.

I'm not a cat expert, but I've had some experience with a dog that really struggled with change. I was introduced to flower essences and have found some relief for myself as well as that dog.

Personally I've used Bach Rescue Remedy for many situations where my emotions were crazy and I really needed some help keeping calm. (Horrible work confrontations. Job interviews. etc) It's a tincture that you drop under your tongue. Within a few minutes I feel better able to remain calm and keep my emotions under control. Really helpful for getting through high stress situations.

For my dog and a very anxious family dog, I have also used the same Bach Rescue Remedy. High stress vet visits or other high stress situations where I wanted to help the dogs feel more calm.

For the very anxious family dog, we have used a variety of the flower essences. For example, when I would dog sit this high stress dog while his family was out of town, we would put some drops of flower essence in the dog's water bowl to help him cope with the stressful situation. (The home where I would dog-sit him was not stressful, but any change was very stressful for this poor pup.) This Bach Remedies list describes the various flower essences and their use.

It might be a low cost, low risk option to put some drops in the cat's water or food to help her manage such a stressful time. I imagine the loss of a companion for some animals must be terrifying. And while I imagine ultimately having some company will benefit her, it's probably also stressful to have a new animal on her turf. (Just listening to people talk about cats I'm making this assumption. I don't have experience with it myself.)

There is now a Feliway for multiple cats, it has calming pheromones like mother cats produce during nursing and is supposed to reduce social anxiety and stress. The original classic Feliway scent is supposed to mimic territorial marking scent which at least in theory might be counterproductive in your situation. I'd get several plug in diffusers and place them around the house; Amazon has multipacks of diffusers and refills. Also research glucosamine for cats prone to urinary issues, if you haven't already...it's frequently prescribed for exactly what you describe as it is thought to help the lining of the bladder, reduce inflammation and help with painful and frequent urination.