BlizzardElixirs now stack in two categories, Battle (Offensive) and Guardian (Defensive) Elixirs. You can only have one of each type of Elixir up at a time. As a result Elixirs now stack with class abilities such as Arcane Intellect.
In The Burning Crusade, most consumable buffs are classified as either a "Battle Elixir" or "Guardian Elixir". You can only have one Battle Elixir and one Guardian Elixir effect active at once on your character. Battle Elixirs are typically offensive benefits, and Guardian Elixirs are typically defensive benefits. Oddly the magical scrolls like Scroll of Agility IV
are not classified as either Battle or Guardian Elixirs, meaning they can be used alongside your Elixir buffs. Swiftness of Zanza
is a particularly good choice for your Guardian Elixir, since it is one of few Guardian Elixirs that meaningfully increases your damage output. Moving faster means you spend less time running between enemies and more time damaging them and thus deal more total damage.