imho: change the hunter to mage.
in my experience, hunters tend to add mob groups rather happily. so after reading your post i'd suggest the following group constellation: blood dk, resto druid, arcane mage, destro lock, shadow priest.

try entering this group into http://raidcomp.mmo-champion.com/ and you'll get, where i'm coming from. this group is a rather optimal cast-damage group with all important buffs and debuffs and caster-only groups always seemed a little easier for me.

just my 2 cents...