The family grey, Madison, lived out the first three years of her life (with us and period since we got her as a puppy) with an elderly female cocker spaniel. Cinda and Madison got along fine despite age, but because Madison was that much bigger than Cinda we did have a slight bullying problem for a few days until it was made clear that such behavior would not be tolerated.
It doesn't really depend on the size of the other dog (unless your grey is not small dog safe), just the personalities. For example, some people just do not get along, for some reason their personalities clash and they can't see eye to eye.
A majot thing to do once you decide and get your new dog is to have them meet on neutral turf, ex. the street.
This is a little farther than you are now, but nice to have in the back of your head:
If you have a male, I would be wary of getting a larger dog for fear that it would want to be the dominant male of the group. (While your male would be used to being the boss.)
Female, be wary of getting a female that wants to be the head hauncho too, especially if your girl is dominant.
If your grey isn't a dominant dog, don't worry about it.
There's my book, haha. I love skirting around the main issue (kidding), but yeah, we had a small dog and she and Madison were fine with eachother.