I saw a YouTube video (I think) where a security cam caught coyotes attacking a small dog. It wasn't a pretty sight. The dog survived, though, but I think it was because a young man came over. Yes, they will attack dogs. Ours are 30 lb or so... like you said... medium sized.
I don't think they'll attack the dogs if they are together, as they are almost the size of coyotes... a little shorter maybe.
But if multiple coyotes get a chance to attack ONE of our dogs by himself, I don't think they'll hesitate.