Hi SweetSunshine,
Most of the independent rescues out there are really good about making sure you get the right dog. They don't want the dog to be shuffled around so they will take a lot of time to make sure you get the dog that is perfect for your family.
The pound isn't quite as good about that, but it is only because they don't have a lot of time to spend with each dog like the rescues do.
Most rescues and the pound will allow you to foster a dog with the option to adopt. Basically what you do is bring the dog in and see how they get along with your dog and your family. If it doesn't work out then you are really only dog-sitting anyway, and you have helped the rescue to socialize the dog anyway. Most of the time it works out for the best anyway and the people who do this adopt.
I have had a contant stream of fosters for the past while now. I keep them until the rescue has found them a new home. I don't intend to adopt them when I take them in though. I just keep them out of the local kill shelters until they have a home. Sometimes a week sometimes a few months. I only have 1 dog of my own now and 3 cats.
It will be really good for your dog to bring in a younger campanion. Nothing brings youth back into an older dog like a young dog to play with

.
I think its a good idea to wait until after vacation though. You want to have plenty of time with your new addition.