Hi Clara,
Sorry for the delay, but I wanted to communicate with some committee members as a Foreign Language guidebook has yet to be published to handle such matters. Our consensus is:
Since the book seems to be all in Spanish, the same rules apply to English and single letters: any letter such as "y" that can be misread as a word in Spanish should have a grade one indicator. So, yes on "y" and no on "x".
Hope that helps!
Patrick