It is, of course, best to introduce them when they're pups. And, I don't want to put a damper on things, here; but.....SOME large dogs have an unreasonable aversion to small dogs. There have been several tragic stories of unprovoked attacks (here, on YT) by larger dogs on Yorkies. And, one of my friends who is owned by a Puggle told me that her pooch's litter mate killed a new puppy that was introduced into the family. The two had been well-acquainted, ate together, slept together for months. Then, one day in the yard, the older puggle snatched up the pup, and shook it like a rag doll. The pup's neck was broken, and it died.
My Mozart is a 12 y/o 25lb mix, whom Abigail bosses around
. He will not snap at her no matter what she does - and, she can deal him some pure misery at times - but, I won't leave them together in the same room if we leave the house.
Like I said, I don't want to put a damper on things; but, I wouldn't take anything for granted, either.