Public drinking is not allowed in Toronto. (though nothing stops people from using a coffee cup with their beverage) There isn't much of a tailgating thing in Toronto, or space to do tailgating around BMO Field. Usually people go to a pub for pre drinking. Many bars walking distance from BMO Field, but I'd avoid ones frequented by TFC supporters group. Even if you and other Timbers supporters go to a supporter's pub, it's unlikely you'll run into grief, but prepare for some good natured ribbing.
IIRC section 104 is reserved for away supporters, but best ask TFC FO.
Enjoy your time in Toronto