Sicycat, this is a very tuff decision and as many have said before, u are the best person to determine this cuz u're the one who knows Zoey the best.
Let me share some of my experiences- some good and bad..
I adopted a calico kitten named Rusty when I was still living at home about 8 yrs ago, and ADORED her. About 3 years later, I was driving home from work and saw a orange tabby sitting near the road. He was sitting too close to the road so I pulled over to shoo him away but as I walked up, he remained sitting (breadbox position) and I realized he had blood in his mouth. To make a long story short, I brought him home from the humane society where I had taken him to because they were going to put him to sleep. I had promised my dad that i'd find another home for him since dad REFUSED to have more than one cat but after a couple days of searching for a new home with no luck, Sammy won my and my mom's hearts so we pleaded with dad to let us keep Sammy and dad finally gave in.
The sad thing is Rusty has not forgiven me for bringing Sammy home. This shows how SMART cats can be. Rusty knew it was ME that brought Sammy home. I tried to give her extra attention so that way she wouldn't feel neglected but it didn't work. Up to about a year ago, she would still hiss and swat at me if I came too close to her and she'd still hiss at Sammy. Fortunely, they have not gotten in many physical fights and now has learned to exist in the same house with each other. Recently I went home to visit for 10 days and to my surprise, Rusty let me pet her without hissing for a while! Rusty has become my dad's "cat" and Sammy has become mom's cat so it has worked out okay..
Positive story- all 4 of my cats adores each other.
Zebra was about 1 yr old when I brought Spike and Buddy home (they were about 3-4 wks old) and she immediately adopted them. It shocked me! I was planning on keeping them separate for a while (I live in a 1 bedroom apt same as u do). Within an hour of arriving home with the boys, I was cleaning out the carrier to get it ready for them to sleep in and as I did, the kittens tottered up to Zebra without me noticing. Zebra started licking them before I could get to them and I was soo surprised! I still kept them in the bathroom and/or bedroom while i was out for the first week but never had any problems. Poor Zebra HATED it when I put the boys in the bathroom or bedroom and would park herself at the door waiting for me to open the door then she'd go crazy licking them!!
Remember that Zebra was already used to having another cat as a companion..
I had rescued a feral pregnant cat from a blizzard in Feb and she gave birth to 5 kittens in my bedroom before I could get her to a foster home. (remember I live in a 1 bedroom apt so that meant I had NINE cats/kittens)!! When I brought the kittens out of the bedroom to introduce slowly to my cats, Zebra immediately accepted all but markedly favored Pepper while the boys HATED all kittens. The boys would HISSSSSSSSSSSSSSSS like crazy... It took about a month or so before the boys got used to having the kittens around. Unfortunately Sweetie (mom cat) didn't like my cats and kept attacking my cats so I finally had to give her to a cat rescue group to foster and try to find a home for her while I found homes for all kittens but Pepper who was adopted by me and my other cats. My cats ADORES Pepper now..
It really depends on various factors in determining whether Zoey would accept another cat. The age of the "new" cat is definately a factor.. I have noticed that hissy has talked about the "bad history" of Zoey and that could be another factor also. (I'm curious about the history of Zoey so could u share that info?)
It's really great that u are really thinking this throughly and doing research before making a decision. Hope this helps..
Another idea is to "foster" a cat/kitten- you can go to a cat rescue group and apply to be a foster mom. They are always SCREAMING for more foster homes for cats.. This might be ideal because this way u can test Zoey with another cat and see how it goes but at the same time it can be for a temporary period of time then if it doesn't work out, u can always return to the cat organization...
About feeding kitten/catfood- I had the same problem since all 4 of my cats are various ages and Pepper is the kitten. At first, I'd feed Pepper in separate rooms so that way she'd eat kittenfood only but now am starting to feed them mixed kitten/adult food in the same room now since Pepper is now about 7 1/2 months old...After she's 1 yr old, I will start feeding adult catfood only to all 4..
SORRY THIS IS SOO LONG! Oopps..