To the OP : The key to nice dps as a frost mage here is to NT everything.
Your mages should know every boss should have NT on him all the time. You'll spend most of your time refreshing NT on them but it's worth it.
Choose wisely when to use frozen orb for maximum cleave.
You can Freeze the adds from Mar'li for an extra FoF proc.
Frost mages should not be a problem at all, we always killed it with me as a frost mage, and my dps doesnt hurt the raid, its quite the other way around : http://worldoflogs.com/reports/utrww...?s=4075&e=4349 - http://worldoflogs.com/reports/gbhpc...?s=3469&e=3809 (those are pretty old but you can see a frost mage can do his share of dps on this boss, and NT is around 30% of my dps)
Despite what some may think, frost doesn't suck.
We never stacked up the bosses because we didnt feel like its a good solution when you do not outgear it. But the key for us was to leave 1 melee on Sul the whole time and interupt his cast whenever possible. He should die pretty quickly and never cast his adds.
The fight has been nerfed a few times so I don't really know if Sul's damage is still a problem.
Sul should die as fast as possible, when he's dead, the fight's pretty much over ^^.
Good luck !