Thanks for inquiring about the process of currency exchange in QuickBooks Online (QBO). You've come to the right place,pennees.
Regarding your concern, every currency has different sequence. CAD won't follow the sequence of USD and other currencies. QuickBooks separate sequences every sales transactions to organize and it would not mess up the books.
Read through the overview of this article about setting up Multi-currency in QuickBooks Desktop: Set up and use Multicurrency.
Also, when you turn on multicurrency, QuickBooks uses exchange rates to get the home currency values of your foreign currency transactions. The system also uses exchange rates for your reports. It converts foreign currency amounts to home currency values, regardless of what is used in transactions.
I'm also adding this helpful article about the maximum number of characters you can enter in each field: Character Limitations.
Let me know if you need additional information by commenting below. I'll be around if there's anything else I can do with the number sequence in QuickBooks.