The last time I had to clear customs in YUL (Montreal), I also had to go through the domestic departures security line. The wording on the airport's website seems to indicate that this procedure has changed in the past few years, and I can remain in the secure area of the terminal in order to make my connecting (domestic) flight. Is this the case? I should also note that on my last trip through YUL, I was arriving from an overseas (and non-EU) country, which apparently still need to go through the normal security screening after customs.
It is likely that I'll also be bringing some duty free alcohol back with me, but my understanding from this page is that, should I need to clear security again, it will not be an issue to have that in my carry-on baggage (I do not plan to bring a checked bag with me on this trip).