Hello, I hope it's ok to not post a picture, I'm quite a mess right now. I lack sleep, and I'm stressed, and just stopping panicking, lying down to write this and sleep.

I just brought home, on somewhat of a whim, the goodest and most communicative little boy cat from the street (he looks around or less than 6 months).

And I already have a lovely, cranky, scaredy cat of a 6 year old lady (sterilised).

I made the assumed mistake of letting them meet face to face as soon as I unlocked the door to the apartment.

Anya (old lady cat) froze, and left, scared, and went to her room with her litterbox, and has stayed there since.

The Boy, I left a bit around the apartment, to make sure he actually wants to stay instead of going back to the street (he loves it), took him outside and back once so he could emergency shit because he didn't like the sand in the second litterbox I prepared (maybe too little, I'll get more tomorrow), and now I've closed him in the WC, with a good-sized box with his new fluffy blanket, food and water for a day, and the insufficient but untouched litterbox. And he is very happy in his box, in the closed room.

But in between all that, were the incidents:

A couple of times, The Boy went into Anya's room, found her, sat far away, curled his tail around his feets, and slow-blinked at her. She was not accepting. Deep-mewls, staying as far as possible (cornered), raised hairs on tail, tail circling around her side not touching her body, lowered ears, wide eyes.

He respected the distance.

I put him in the bathroom, closed the door, put her food outside the door to the bathroom. Left him to relax (he is, he is fine). Sat with her in her room for half an hour, so she would relax. She did a bit. Still hasn't left her room, but came for pets after a while, in her room.

So. Given how unprepared I am for this, and how much I love both kittens, what tips do you have for me? What to be careful of?

The situation is not ideal. Is there any condition, where I should be prepared to send The Boy to be an outside cat, for the better of everyone?

I'm dying of sleepiness, ask anything, and any help is welcome. Thank you so much for even reading all this. Now I'm gonna sleep, and I'll be back in the morning.

He is a very good boy, smartest and most communicative and trusting cat I've ever met, and even after the messy vet visit, he still repeatedly chose to come back with me whenever I gave him the choice of returning where I found him, or even just outside the building. No, he wants to be here. That fact helped me calmed down a lot.

Currently, the boy has gorged himself full, from the stress of the vet visit no doubt, and is contently sleeping in his box in the closed bathroom. Anya is walking around the house, generally smelling everything, and paying occasional attention to the bathroom door. She is eating and drinking normally every so often, eyeing warily the faraway but visible door. She no longer looks actively scared or stressed, looking ok enough with the current arrangement.

I read Jackson's guides, and they've been really helpful for context and building intuition, but neither my house, nor my situation allow really following them step by step, so we're gonna wing it a bit.

I'm going to have each of them inside their own room (different rooms) for a portion of each day, leaving the other one to roam the entire house for that time. Anya evening to noon (play->sleep->job->relax) time, the boy the rest. I'm lucky I have 5 days off work starting today, to give them the attention they need.