Yes you can use your big unbrella I'v seen them there before
As for a good spot to set up, I like the area between turn 3-4 on the outside, from there its just a short walk down to Moss or you could go the other way and up to the top of turn 2, great spots to take photos
Moss corner 2007
corner 2 2004
But realy theres not a bad spot at Mosport to watch a race from