Again, I hope you can find the rightful owners and as I said before, help them get her spayed - you know - convince them.
If not hopefully you can afford to keep her. She sounds like a doll. And it sounds like you shouldn't have an issue trimming her nails. It also sounds like your elderly cat is not really having an issue with the newcomer and vice versa. A little hissing is nothing. Keeping her sequestered in your bedroom is also perfectly fine. And I don't think you messed up on the intros because of what I said - there was no serious posturing or aggressive behavior. "Nakita" as you named her, sounds very un-territorial.
Good luck. We are all pulling for you and your little foundling.
Oh what pretty markings! She looks like she really enjoys being around you and even the other cat. Sounds like a pretty easygoing gal! I hope you can keep her or find the owner. It sounds like she'd like to keep you guys for sure!
I have a almost 17 year old cat and she sleeps with my 6 month old kitten all the time. Try to keep her if you can. I would hate for her to go to the pound.
Just a little more info on Nakita. I took her to the vet tonight to get her checked for a chip. She does not have one. They also said she is about 10 months old, she has hee big teeth. They took her out of the carrier to scan her and they said she responded to scruffing really well, they also clipped her nails for me. She didn't mind at all....LOL