With a dedicated healer already on the team (Disc Priest), you won't often need any support from the Shammies.
If you like the damage or features (food/portals, etc) of the mage, no need for more Shammies.

Two totems gets you most of the buffs you would want for an all caster team (Prot Pally uses mana, benefits from spell power -- although Str is converted to SP so they don't need to stack it at all.).

I would consider a dual spec on one of the Shammies, to Resto. You won't need two healers all of the time, or even most of the time. But you'll have the option of Resto Shammy or Disc Priest as the healer, or possibly both if you felt an encounter was really challenging and not a DPS/Enrage race.