I don't believe it. They're in the same room, no fighting, Nemo's smart enough not to get to close and they seem to be doing fine till now. They're both lying on the ground, asleep, a few feet apart! (I hope the vet did a good job getting my cat fixed, cos she isn't. I mean, hope he snipped em both off yes I'm a bad person, I know).
Thanks for your help and suggestons so far!

I had a simailar situation. I asked the vet, and he just shrugged and say they'll get use to it. My first cat, is older...and she didn't much like the new younger kitten. He was male, but he was rather startled/jumpy since I found him on the street. He would even hiss at me during this time. My old cat would hiss at him, and growl, and he would shy away. But later on they got along fine, it'll take a while...time will tell. I do recommend giving the new cat his own special place for a while, that way he'll seem less intrusive. Or she for that matter. Like he's not inruding on his/her territory.

yeah I was kinda worried about that too Sikku (sorry cannot write your full name 'cos of what it means in Dutch ).
Thanks anyway.

Honestly, when I tried introducing a new cat, it was a very bumpy ride. It took about a week to get to the point where they'd coexist, and they never much liked each other. Right up to when the new cat and I moved out, the old cat would just attack her at random, and the new cat was stressed all the time. Some cats just don't do well with other cats.

One thing that helped a lot is a little plug-in we got with cat-calming pheremones. We got it from our vet for $40 and had to refill it every month or two. 'Spensive, and it felt a bit like we were drugging them, but it helped just enough that it was very, very worth it and we didn't feel too bad about it. Something to look in to if the problem persists. I believe one such product is called "Feliway," which doesn't make sense to me because it's not actually a deterrent. But whatever.

thanks Mai!

I have been through this ordeal a few times myself. Sadly, there's not really one solution, because as you know all cats have VERY distinct personalities. Id try putting the cat that is use to the house in a carrier so that the new kittie doesn't feel so scared while checking out his new place. Let him wonder around or cuddle up to you for a few hours without the other one in sight. Once new kitty has had a chance to get sniff around let out the other one. I know it sounds awful, but you may just have to let them fight it out at first. Just keep picking up and petting the new one when he gets upset (if you can do so without being clawed madly) and he will calm down. Hope this helps a little. Good luck.