Adding a second dog to the household, any advice?

Things to be concerned about;

Katie is a female I'm assuming. Introducing another female dog could be a little tricky, especially if both haven't been spayed. Females tend to exhibit more resource guarding tendencies than males. It could cause problems.

Katie is only a year old, and still a puppy mentally and physically. A new dog may confuse her and she may revert back to puppy behavior to keep the attention on her, she may also begin a pattern of negative behavior to recieve attention of she feels the new dog is getting too much attention.
There is a lot of difference between a year old dog and a three year old. Unless Katie is very forward and dominant, a three year old could easily intimidate her too much, which would be bad for everyone involved. I would really try to keep that age a little closer together. If Katie were the three year old I wouldn't be as concerned.

I would ask for the rescue/adoption to be on a trial basis for a few weeks to give you a chance to see how they interact with each other, and make sure both dogs are up to date on all shots. Look for food aggression behaviors and over dominance issues.

I have two females. One is just over two years old and the other is just over a year old. I got the two year old this past May, and the year old this past August. Mine knock the snot out of each other playing and mock fighting, steal cookies from each other, and then sleep on top of each other at days end. It can be done.

We added Virgil to our household a year ago because our 11 year old vizsla cross was looking, well.... old. Victor the cross took the addition quite well and enjoys playing with Virgil some times. He has taught Virgil some good lessons on who's boss and how to behave. The older dog likes to sleep a lot and when they go out to play, plays hard and then needs to sleep again for a while, so Virgil is on his own alot.

Our dogs also live outside. They play together part of the time when Vic wants to, but we also have 9 cats for company, cattle to herd and ducks and chickens to observe and keep out of the yard. Also we farm and there is usually someone around outside 25-50% of the day. So there is a lot of entertainment for Virgil who is 17 mos old.

I think that when the time comes we will have to seek a companion for Virgil. It did a lot of good for our older dog when we got him a friend.
If no one has been around I will bring Virgil in the house in the evening when I have time just to sit and pet him. No playing in the house hes too big. Our dogs are both neutered males.

I think you need to consider the energy levels of companion dogs. Virgil would have a lot more fun if Vic would come out and play more. Virgil just glows when Vic is out playing and stands and looks at Vics house like I'm not done playing yet when Vic has went back to bed.

We have an 11 yr old V and we're getting an 8 wk old V pup next week. I'm hoping the pup will push our old fella to well.. not be so old! Must admit I'm a bit nervous as Bentley is much less patient with younger dogs than he used to be, but I'm sick of him sleeping all day!

But back to the original question: I do dog rescue where I foster dogs that are death row and find them homes and recently I rehomed a male pointer x who was 10 mths old to a home that had a 2yr old female Kelpie. The new family had probelms with the dogs not getting along for a few weeks and kept saying the dogs couldn't decide who was dominent.

After discussing exactly what was going on, I discovered that the family really wanted the Kelpie to be top dog cause she was there first and their actions reflected this. After I suggested they make the new rescue dog top dog, everything changed! Within a few days the dogs were best friends and have been great together since.

I guess my point is when you introduce a new dog, you'll have to suss out who you think is 'alpha' and support that decision, even if it doesn't seem fair. The dogs will be so much happier knowing where they stand, especially if one is naturally a leader and one a follower..