There are always happy endings.

That's the rule. They may have to suffer to get to it, but at the end of the day, our little ponies are happy. Maybe there's a little bit of bitter mixed in with the sweet, maybe even a lot of bitter, and that's certainly allowed here, but anyone in the story should be able to look back through their adventures and say, "it was worth it", despite whatever pain they might have gone through in the story. There may be problems at the end, but nothing that can't be overcome. Shipping is welcome. Anything is welcome, as long as you can say at the end, with a straight face, 'and they all lived happily ever after'.