Make sure you have time to fill out the rubric completely. Canvas won't let you change your rubric ratings once they're saved. You can leave a comment indicating changes you want to make. If the rating factors into the other student's grade, you may want to message your instructor to let them know about the mix-up.
Media comments and attached file comments display in the same list with text comments.
Contact the DoIT helpdesk if you have further questions about using Canvas.