Old dog meets new dog. Help?

I have a border collie dog named Rex. He is six years old and we got him when he was one year old. He is my dog but my dad is the 'leader of the pack.' Rex has never been known to be aggressive. If anything, he is more excited. He is a hyper dog.
Rex has free roam of the house and sleeps in my bedroom at night. However, my cat has been here the longest and is the ruler of Rex.
We got an 8 week old border collie puppie. We brought him home today and what we have done is we first put the puppie in the kennel and let them meet behind there. We put him in the unfinished area. Rex owns the laundry room so we figured it wouldn't be best to invade his area. The cat has met him and didn't really mind. He got fluffy and growled a little bit, but my cat always growls.
I need as much advice as I can get. My dog was getting tense and attempted to nip at the cage. It was sudden movement and almost herding, I think.
I need any advice that you can give, it doesn't matter what it is.

Oh, and ever since Rex met the puppy, all he wants to do is play with it. He used to be so co-dependant and now he barely notices me. Is that normal? Will he go back to the way he was?

When I introduced my Rott mix to my parents new dog he was upset too. He does great at the dog parks and loves meeting new dogs on walks but for some reason didn't like my parents Dog. So I was told by my vet to take them both on a walk together and take a couple other family members and walk them with the Older dog leading a little more in front of the new dog but still whoever he considers the leader of the pack to be in front. Since dogs are pack animals this is kind of an initiation into the pack it will show Rex he is still in the pack and excepted but that he's still not in charge. They will snip at each other once and awhile on the walk but just kind of keep a tight grip on the leash and warn the family to wear good tennis shoes cause you'll need to go on a long walk. I did this a couple times and it was done. They haven't had a problem since...minus a misunderstanding about who's bone was who's once. but otherwise they sleep together and play together. Try it it worked well for me!!! =) Good luck!

Check out what I attached from Cesar Millan (The Dog Whisperer) about introducing a puppy to an older dog.

Go ask a vet. This is really unusual...

it would have been best to introduce the dogs on completely neutral territory.

pay close attention to how the puppy reacts to the older dog.
rex may be reprimanding the pup for poor doggy manners. when an older dog scolds a puppy, it can look pretty scary -- growls, snarls, snapping. so, how do you know whether the older dog is AGGRESSIVE towards the puppy, or if he's SCOLDING the puppy? you must pay attention to the pup's reaction. if the dog is aggressive, the pup will most likely fear him and behave timidly around him. if the dog is scolding, the pup will most likely display very convincing submissive gestures (like rolling on its back, whining, licking the adult's face and looking like its begging for its life!), and then bounce right back into normal behavior! that's the key to determining what rex was doing -- how the puppy reacted.