I'd consider a few more Hybrids.
You can spec a Shammy both Elemental or Resto.
A priest can go Shadow dps or Disc/Holy healing.
A druid can go Boomkin dps or Resto healing.

That gets you a bit more flexibility.
Some fights might be doable with two healers, others might require three.
If a given boss does not Enrage, you could go with three until you outgear the content, then use two for additional speed.

Take a look at the buffs you get, both the spells and the procs.
Take a look at the debuffs you can place upon the mobs.
Try to have as many of these as are relevant to you.