my new rescue feral not leaving her cube

Hello all cat lovers,

I'm new to this site. 

So I took in a 7 month old cat that was feral for the first few months of her life and then in and out of homes. She is vert scared. She got spayed yesterday and I picked her up from her rescuer last night. 

I have her in my bathroom with a bed cube thing. She won't get out of it to eat, drink or potty. She has not pottied in over 24 hours! She was litter trained in one of her fosters. She has never attached/bonded with a human but she has with other animals.

I have fed her wet food and she has licked it up but I don't think she really chewed anything.

When she first came in last night she was comfy in bed.. she seemed fine and i made the mistake of taking her in my living room. My senior cat sniffed her the first couple of times but then sassed out and hissed.. Recently in September my senior, Indigo, and I lost our fur baby child sweet love, Jack. They had been together for 11-12 years  of her 15 and his 13 and they acted like brother and sister but still very independent . 

Indigo does not like other animals , hisses at my friends with dogs that come over and then just ignores them. She is the sweetest thing.

So my questions besides just telling my story is how best can I get him to trust me? He isnt drinking water or going potty, I dont even care if he potties anywhere just worried about his health... question part is when do i start to really worry about him being to scared to use litter box? 

Should I bring him into my closet when I take a shower later? 

Right now I have door shut , lights off , slow harp music playing(I read ferals like the harp) and I go in there to hang out with him and talk to him every so often and obviousy since its my only bathroom, I go in there to use the bathroom etc. 

the poor girl is burrowed in her bed under her bed cushion.

I want what's best for Indigo and her.... She doesn't have a name yet, I mean the girl that originally adopted him and then TOOK HIM TO A KILL SHELTER, named her Misty but i think good riddance to that name and that sad life. I want her to feel my love and trust . OH and I would love name recommendations. Right now im kind of calling her Sweet Pea.

-Allison 

Awww, good luck with your new girl (I think it's a girl, you said he/she a couple times so not sure), but my experience introducing multiple cats is it needs to be done very, very slowly...and it sounds like the one already that has run of the house isn't very friendly. Do not rush this - it should take days and maybe weeks, especially with a feral...after a bit you could exchange blankets with the 2 so they can get used to each other's scent.. As far as not using the box for pee or poo since having surgery (but has eaten)....I might call the vet to check about that. She may have peed in the cubby she's in, too afraid to come out, but if you find that's not the case, check with the vet that did the surgery. My vet has mentioned to me that she is amazed how long cats "hold it", but I've also had my old cat pee in the carrier when she couldn't hold it anymore. Cats hate that! I would maybe give the music a rest, let her get used to the sounds of where she is through the door. If it's nice and quiet maybe she'll come out to use the box. I'm sure the poor thing has been really traumatized with the moving around, being in a shelter, and then having minor surgery. Sooo much patience is needed with a feral, but they can end up being the best cat! I adore my tamed feral and she's the most affectionate with us!