Thank you for adopting this rescued this kitty!
Do you know how old the kitty is? Did the vet have any idea?
As to adopting another kitty, it might be a good idea for your morale if you adopt one you're sure is people friendly. I don't know how much it would actually help Rockstar. There is just no way to know in advance whether cats are going to be friendly with other cats. After our first rescue, we rescued a second one so he had a friend.
It is all too apparent that he would have been perfectly happy as an alone kitty. We have six cats, and just brought in a 7th that we're fostering - and he was REALLY close to his brother, misses him like heck, and desperately wants to be friends with one of our cats. Our kitties tolerate each other and share their space well, but just don't really play with each other and don't snuggle with each other anymore (they did as kittens). So our new foster is bumming about everyone hissing at him (even with the slow introductions).
Right at the moment, look at your Rockstar this way. Think of him as a four-year old child that was on vacation with its parents from the U.S. in China or something. They got separated, the child got lost, and someone hit him with a car, he had to go to the hospital to have surgery, tests, and got poked and prodded in a terrifying environment. Some Chinese family adopted him and took him home. He is TERRIFIED. Everything smells different, everything sounds different, looks different, smells different - it's not his bed, he doesn't recognize any of the toys and doesn't know what to do with them. Two weeks is a very short time to expect this little child to love his new parents and to feel comfortable in his new home.
To cats, territory is the most important thing. That's why it's important to start with the small space. If you can, I would recommend moving him into a bedroom or something. He needs a place to hide (a box on its side is good, under a bed is good, under a table with a cloth over it is good.... ). A night lite is good so it's not completely dark at night. A radio tuned to a classical station is very calming.
Purchasing Feliway and using that helps to reduce stress. It is a synthetic pheremone that mimics the "friendly" markers in cats' cheeks. Spray it around at cat-head height - but not near litter boxes or anything you want him to scratch on.
Spending as much time in the room with him as you can - but IGNORING him is important. Read out loud, work on a laptop, watch TV (not too loud), sew, fold laundry - do whatever you can think of in there. Just let him get used to you.
Keeping a routine is VERY helpful. In socializing cats, what you are doing is earning their trust. At first, you just have to establish that you don't want anything from them, and you're there to give them stuff they need.
Cleaning the litter box, cleaning and filling the water dish, filling the food dish, and serving a meal of breakfast, lunch, or dinner at the same time every day really helps accomplish this. And just being in the room, doing something else, is very non-threatening, and gives them time to check you out and get used to everything.
Food is also a great motivator. Baby food (Gerber all meat, chicken, lamb, veal or beef best) held out on a spoon can be a good thing. Sitting on the floor with your side facing kitty holding out that treat may encourage him to come out of his shell.
Also, don't look Rockstar directly in the eyes. This, to cats, is a sign of aggression. Look at his forehead or over his head.
In fact, "looking" at him with your eyes closed is a great trust builder. Sit on the floor with your side to him, close your eyes, and slowly turn your head and "look" at him with your eyes closed for 10 - 20 seconds, then slowly open them. After time, you may find he "slow blinks" back at you. All of our kitties do the "I love you" (I trust you) slow blink - several will keep it up for a couple of minutes, and if we wink just one eye, they'll wink just one eye back.
Your kitty doesn't hate you. He's been completely uprooted, traumatized, and doesn't understand what's happening to him. If he was loved, he was lost or abandoned, and so you've got that trust hurdle to cross too.
Just give him some time - he'll come around.
Now none of this is to say you shouldn't get another kitty. But if you adopt a "user friendly" kitty, he'll make the territory of your home his, and then this kitty will have that introduction/integration barrier to tackle as well. That's not necessarily a "bad" thing, just something to consider.
Hope this helps,