a) Only Items that drop from mobs that are equipable are added to the transmog.
b) Quest items are only added if they are equipable. If an quest gives a Cloth Chest Piece, mail legs, or a 2-hand Mace; a mage will only unlock the chest piece. however a Paladin will unlock all 3 items.
c) If a quest reward is spec specific on rewards (leather gear for monks) it will only unlock the items for that spec.
....1) Only caveat are weapons. If a weapon reward is given, it will unlock all weapons that class can use even if it is not for that spec (Windwalker monk reward is polearm or sword; but Mistweaver reward is 1-hand mace or staff; it will unlock all 4)