AC gives 60 minutes MCT for Canada-US connections at YUL, and in any event I'd agree that 57 minutes is cutting it extremely close considering how quickly a few minutes of delay can compound. Picking random October days for YEG-ATL itineraries on AC's website, I see one 60 minute connection through YUL that must be on the last YUL-ATL flight of the day. I would reschedule if at all possible.