For my mixed classes, I deliberately picked instant casting classes (warlock, hunter, warrior, priest). But, if your mage has to do something every so often, just put it in a sequence for him.
/castsequence [target=focus-target] reset=target/25 Scorch, Fireball, , Scorch, Fireball, ,
The extra comma is to give the char time to cast the spell, other people will be doing something instant in that time and if you just mash the button with no pauses, the mage will be out of sequence of other people. Bascially it gives the mage time to cast the spell and stil be on the same step as other people.

An example:
/castsequence [target=focus-target] reset=combat/target Sunder, Demoralizing Shout, Thunder Clap, , , ,
/castsequence [target=focus-target] reset=combat/target Corruption, Curse of Agony, Immolate, , !Shoot, , , , ,
/castsequence [target=focus-target] reset=combat/target Shadow Word: Pain, , , , !Shoot, , , , ,
/castseuqence [target=focus-target] reset=combat/target , Serpent Sting, , Arcane Shot, , , , , , , , , , ,

You can try to match up spells. You can put in pauses for alts so they wait for the tank to get aggro, or the healer to catch up or anythng. Most things outside of an instance, just do 1-2 spells, most mobs will die too fast for anything else. Hope that helps.