In the spirit of my experience reading this comic strip, you are in a split fiction where your comment is with the fiction in the next comment. Do with that what you will.
As long as you keep your fictions brief along with being easy to imagine and understand, you can hop in. Don't be writing the whole lore of your fanfic or making the fiction incomprehensible that the previous person doesn't understand what the fiction's implications are. If a reader can't comprehend it, then it's the next fiction over that becomes cannon. I expect you understand what the consequences are if you break any other rules. Have fun or else. >:)