Honestly there could be an alternate event that leads to the two meeting under different circumstances. One where Toffee saves Moons mother from getting killed or something. From there a romance is possible but i'd bet it would probably be some event that allows them to meet prior to the events. There are a lot of different outcomes but I can't think of them because we don't know much about Toffee's past, then again we only know Moons mother was killed and she took over at Stars age. Anything prior is unfamiliar territory. If we knew more it could be possible, then again empty spaces are great places for fan-fiction because you can use your imagination and you can do anything with the story.