Hi Shannongaia
I am not a vet or rescuer just an observer lingering around looking for a dog to adopt, saw your post and couldn't help myself I had to reply..........
I have 8 cats and have learned alot about their behavoir and am still learning thanks to this site........
I may be wrong but I found it strange for a vet to tell you to put both cats in the bathroom together,
In my experience I have always let my new arrivals have roam of the whole house to check out their new pad. And when I was not home I would put new arrival in bathroom or other room with litter and food for it's own safety and other cats until I was sure that they could socialize together.
Your older cat sounds like it will adjust just fine to it's new surroundings, the younger one seems to be a little territorial and perhaps agressive, but wait, this world is all new to them and it will take time to settle in , and things could change. If the young one is the aggressor and you see it about to happen that is when I would pick it up and put Him/Her in the bathroom lights off to calm down.
I have found that some cats just need time to get to know each other, as they are both new to each other perhaps little guy feels intimidated and scared maybe after being in your home for a period of time he/she will become more relaxed and at ease......
As I said before I am no expert so perhaps someone with more experience will have better suggestions
My 2 cents