Hi Sophie! Welcome, I’ve added you as a writer.

To answer your questions:

1. Yes, Canadian spelling is allowed so long as it’s easy enough for other English readers to follow along and understand your writing.
2. Yes, stories published elsewhere can be submitted to the publication so long as you set up the canonical link to the original story (see how to do that here: https://help.medium.com/hc/en-us/articles/360033930293-Set-a-canonical-link)