Well, when I first brought Bella home, I confined her in the living room section of my living room/kichen area. I had to stick two framed posters over the doorway, though, because there isn't a door. She was pretty ok with just that one room for the first day, but that night, I slept on the sofa, because she started crying when I left the room.
The next morning, she got the kichen and bathroom. And then, that evening, she hopped over the poster I placed across the doorway to the bedroom (the door won't stay shut). I figured that she was probably fine, and she was. I did leave the living room light on, at night, though. I don't know if it helped her find her way back to the box at night, but electricity is a flat fee included in rent, so it's not like it hurt me.
As soon as I got her home from the Humane Society, I put her in her litter box. I actually didn't have a bag of food in the house, so I gave her some water, and after about 15 minutes, I ran to the store to get some.
My stray is a different story, though. I had to keep it separated from Bella until I could take it to the vet, and then it had to stay in the bathroom because Bella didn't like it. It was stuck in the bathroom for about a week, with short visits to the kichen/living room when I could trap Bella in my bedroom and stick something heavy against the door. At some point, when I was allowing them to be together, one of them got the door to the bedroom open, so now they both have free run of the whole place. All 700'ish square feet. I'd recommend going slower if you have a bigger place.