I'd worry more about the traffic getting back from yorkdale than anything else - or square one for that matter since it will be rush hour

I take the ttc all the time to the airport and its not too bad because its only going a short way, the 401 from the airport to yorkdale can be chock-a-block anytime during a regular commute and that would probably be about the time you're heading back to the airport. That said if you give yourself enough time it probably isn't too bad.

actually looks like there is a go bus to square one from the airport:
http://www.gotransit.com/publicroot/...station=&new=Y

and one to yorkdale:
http://www.gotransit.com/publicroot/...station=&new=Y