The mages do not need to spec specifically at all to aoe. For a team with a tank like WPMML you need at most Arcane subtlety 2/2 for the mages. From there, you can just about pick any spec you enjoy for leveling and boss killing until around 50+.

The Warlock will be pulling aggro constantly unless you give your tank time to accrue enough threat before starting to aoe. Though, that's not preferable because sitting around doing nothing is not efficient. Hellfire has no threat reduction unless you spec full demo, and it is the highest dps aoe in the game. Going deep destruction imo, syncs best with this comp, you can pick up talents like Intensity and Pyroclasm, while high damage shadowbolts sync nicely with the mages bolts and balls. Just be sure to precast bubble-renew on your lock for every aoe pull, and again during the pull, and keep an eye on their health because you'll be splitting tank duties with the warrior. You can also use void sac when it's off cooldown.

A pally healer would make life so much easier, but most people enjoy the aoe heals with the priest, and some go horde.