Good advice Rainy.
I have the same breed mix as the OP. I also have a foster shih zsu that would not bark, snarl or growl if his life depended on it....
Now fast forward to 9 months later - the shih learnt undesireable behaviour from the min/chi. We have been working on not one but now two dogs that feed off of one another. Having 5 dogs in total, it takes one little terror to get the whole pack going consisting of dogs from 5 lbs - 155 lbs.
Until you get this behaviour under control, do not bring in another dog. Things will escalate and you may be dealing with 2 dogs with bad behaviour rather than all concentration on one. Take it from someone who knows only too well.