1. Can I do some tasks with the five short posts and some with the three long posts?

 Yes, you can mix and match the long and short posts as long as the task is done as either all long or all short.  This means a task must be completed in either the long or short.  So, say you do task one as five short posts and task two as three long posts.  That is fine.  But say doing in task three you post one long two short and another long post that is not okay.  Keep the post length the same per task.