Does internet data normally subtract from the normal 15gig shared data when using in Canada?

@Ellisnet No it costs you an arm and a leg like international roaming! Unless:

• You have Canada Roaming Bonus - Where you get 1GB of data with $15 charges for each overage.
  If you have Mobile Share Value plan you can get Canada Roaming Bonus for free, but you have to call and ask for retention to do it (if they don't know what you're talking about, call back and ask someone else) and this gives you 1 GB free. Ask super-sweet as it is NOT part of your plan! 
• You have North America Roaming - Where it works normal. (This is rare to have)

You didn't specify a plan but I'm guessing MSA or MSV...

Cellular data use in Canada does not come out of your 15GB shared data. If you have free Canada roaming on your line, I believe each line gets 1GB of roaming data. If you do not have free Canada roaming (if you never asked for it, you probably don't have it), you will pay international roaming rates for all data use.