The domestic-Int'l transfer through YUL is very easy, so you could be from gate to gate in 5-10mins. The weather is another matter at that time of year. Flights don't get cancelled a lot, but you can expect to have to de-ice on many flights. To be safe, I'd want to take a flight out of YTO that gives you about 90 mins cnx time at YUL. Better to wait there than be stuck in Toronto.