I JUST went through this and I remember how anxious I was...I felt like it was never going to work out and I was completely stressed. I got two new kitties at the same time ---but from different places so same adjustment stuff...and someone told me the same thing other folks are telling you...1) make sure that your kitty has their own safe place the others can't get to (e.g. own room) for a bit and gradually introduce them to each other...a lot of times they will "play" / "explore" each other under the door (sniffing and reaching out etc) which is a nice gradual way for them to get to know each other ... and 2) that it would TAKE TIME...I had the same expectations you did - that it would take a week or two...well, I think it took about a month - 6 weeks before the growling and hissing completely stopped ...(probably about 3 weeks until it even decreased).

Also, I recommend that you spend some extra alone/cuddle time with your kitty to reassure him/her that even though the environment may be different that she/he can trust that your relationship/bond will not be compromised! Mine are doing great but are still working through some of these issues (I've had them now for about 2.5 months) so hang in there and you WILL see progress...PROMISE