IMO, expression packs should really be a base for building up your models final facial morph.  You are correct, some packs completely overwrite your hard work forcing you to restart your project.  Others are just not enough to be realistic.  I usually start the basics and work my way up tweaking settings as I go.  The best source of learning how to create expressions are photographic images.  Your own photo album will contain every possible expression imaginable because they are frank and unposed.  Also look in your local newspapers and magazines.  Some powerful expressions can be created from those photos any day.