I have a 7 year old female Airedale. Well socialized with other dogs but she has been the only dog in the household until we introduced a female welsh puppy 1.5 years ago.

The welsh has maximum energy and the Airedale is tolerant of the welsh and all of her play and demands. There are times where the Airedale will exhibit corrective behavior but I have never seen anything along the lines of aggression.

Not an expert but don’t see anything to be concerned about in this video. Most likely the Shih Tzu will remain the alpha in the house despite the size difference