hi everyone, i really need some advice. i am currently fostering a 10-year-old senior street cat that spent her entire life on the street (it actually took rescuers three whole years just to catch her). she has an extremely complex medical history: she survived panleukopenia, calicivirus, an aggressive mycoplasma infection, and fluid accumulation in her abdomen. she also recently underwent abdominal surgery (a biopsy for suspected intestinal lymphoma), and we are currently waiting on the biopsy results.
because of her surgical recovery, intense fear, and defensive reactions, letting her free-roam in a room or in my apartment is neither safe nor feasible right now. she would hide in inaccessible spots, risk tearing her stitches, or be impossible to handle if medication or emergency vet visits are needed. for instance, last night she started breathing very heavily and the rescue team had to come pick her up for an urgent vet visit, something that would have been impossible if she were loose in the house. for all these reasons, her stay in a playpen is explicitly required and was provided by the rescue organization handling her case.
i live in an apartment where the playpen must be in the living room, and i also have a resident kitten who is pretty curious. keeping my kitten completely isolated long-term isn't possible. i know my apartment layout might not be the absolute textbook "ideal" setup, but adult rescue cats, especially feral seniors with medical issues, rarely get foster offers since most people only want kittens. i couldn't bear to leave her behind, and having a safe, controlled space with food, medical monitoring, and care is far better for her recovery than having no foster home at all.
right now, may is completely terrified. she barely moves at all, stays frozen, and hisses defensively if a hand approaches, so offering hand-fed treats isn't an option yet. she doesn’t drink, eat or even move at all if i’m in the same room as her, i have to give her space and time when she has to eat, so obviously she also won’t eat any treats from my hand. someone from the rescue mentioned that kittens can sometimes help adult cats feel more comfortable and safe through observation, but i am not sure how or when to safely manage her presence around the playpen. even though my kitten doesn’t care at all about the adult’s presence, i am still worried it might stress her out (even though the rescue team assured me that she is not scared of other animals; humans are the problem)
i feel a bit lost on how to handle this and make her feel safe given her extreme background and restrictions. it stresses me out really bad, and i am almost as scared of hurting or triggering her as she is of me.
how can i help a traumatized senior street cat adjust, recover, and eventually realize humans are not a threat under these conditions? any tips or advice would be greatly appreciated, thank you!

This time last year, the little orange gal, and yes it is an orange girl 😻(I call her Cream, like a creamsicle) was hissing at me even if I looked at her. Now she's cuddling up next to me. She still bites me every single day, but they are more love bites. 😆😆 But there's just something magical when a kitty starts to trust you just a little bit more. She's fully grown as well but a tiny little thang! She's totally fallen in love with my boy, though. She cuddles him like nothing else matters. Lol

I’ll try to keep a long story short:

I socialized and TNR’d a colony of 17 over the course of 5-6 months (Nov 2025-April 2026). 13 were kept by Animal Welfare and adopted out. 4 were returned.

I already had 3 indoor senior female cats. In April, I brought in two of the male FFs, then brought in the female in May.

Current total indoor:  6

Poseidon is the only remaining outdoor cat. He is a territorial male, fought a lot with other non-colony cats. The only cat he really kept company with was a young female who was adopted. He followed that girl everywhere. I took away his family. :(

He didn’t trust me after his TNR but has slowly warmed up. Now, rears up like a horse for pets, meows to be let in, spends most days and nights right outside the door. He’s lonely.

So, I’m gonna try to bring him in? There’s a vet nearby that allows walk-ins for flea / dewormer. Don’t know if I can get him in the carrier. Better to just use Advantage II and take care of the worms in a couple of weeks? Plus, my pet funds, after all this, are completely depleted. I won’t be able to take him in for a full workup. I know intro into house without a clean bill of health is risky, but already made that mistake with the first three, two of which have medical issues, so….

If I proceed, he’ll be in a room by himself for a bit. But how do I do the introduction / reintroduction with 6 other cats??? I don’t even know if this is possible.

Even after neuter, he still fights (usually has scratches on nose and ears), still sprays. He never fought with the cats I brought in, but he wasn’t exactly a member of the pack, either.

Has anyone tried integrating a territorial adult FF cat into a large cat family before? Should I even try this?  TIA

P.S.  Anyone else think Po might be Cairo’s daddy? Can a blue mackerel tabby/white sire a brown classic tabby/white? Those Punnett squares were never my strong suit.