You're having trouble with the 3rd/4th set of small adds I'm assuming, but by then AT LEAST 4 DPS should be on those, not just your ranged.
I'm a mage, and to be honest when I went as frost it was hard because my DPS was low and I was supposed to duo the small adds with a Hunter, but if the add puddles aren't straight they'll go all over the place and my freeze moves wouldn't work well.
I'd say to make your mages go one arcane and one fire. Fire should help with adds immensely and Arcane will be able to help with the adds with Blizzard, and still do good single target DPS against ChoGall.
If you could have posted a World of Logs that would have been nice lol.