Can i get another dog if i already have a West Highland Terrier?

My westie is almost 9 and he is starting to slow down a bit. For this reason my parents think its a good idea to get another younger dog to try and speed him up a bit. Also because we want another dog. Im not 100% sure because im worried about what it will do to our original dog. He is very territorial. We would probably get a female rescue dog either a young pub or else a few months old. Any opinions??