Here's what I did to introduce the new guys into the household. (We brought Apollo home when Zeus was 6 months, and then we just brought Onyx into the house, he's 8 months, and the boy-os are both almost 2 years.)
When you go to get the pup, take along a blanket or other piece of cloth that has Riley's smell on it. Allow the pup to lay on the blankie on the way home. This gets him acustomed to Riley's smell, and makes the transition a little easier.
When you get home, keep the pup in a nuetral spot. (out in the driveway, or on the sidewalk) If Riley is *really* territorial, go further afield, like to a park. Once you are ready (i.e. pup is on a leash, done his potty business or whatever) bring Riley out on a leash. Let Riley set the pace of approach, don't let him charge the little one, but if he moves towards him, let him get close. Watch for signs of agression, but allow them to sniff each other out.
If Riley shows signs of getting upset, or overwhelming the pup, back them off of each other, get everyone calmed down, and try again, but more slowly this time. More than likely, Riley will bark; as long as he's not flagging, or showing other signs of agression, it's ok. It'll take time, and patience, and it won't happen overnight, but if you let them set the pace, and take their time, there shouldn't be any problems.
The above advice given is also great, especially about paying attention to Riley and his needs first. He's the Alpha dog (although that might change down the road as your pup gets comfortable and begins to develop more of his personality) after you of course (!), and you don't want to make a bunch of sudden changes in his established routine. This will foster resentment towards the newcomer, and that can lead to bigger problems down the road.
