I am also from Toronto, I would suggest looking at Winners or Marshall's or even going to a local Value Village. I have found several shirts with French cuffs at all those places. I have worn a light blue shirt several times with Highland wear. I like Jock think that a blue shirt and blue everything else may be a bit much, however I would like to see photos of the outfit with both shirts just to make sure.

A coloured shirt will not make it look to casual, remember this is day wear we are discussing. One question I do have is what colour hose are you planning to wear? Another question is where is the Burns Night? I might want to go to another.