Well, we have 4 Healers, one Tank and 5 DPS.
If you go for a 2nd Tank, a DK would be the tank of your choice, because he can dps a little bit during p1&3. otherwise, you would def go for 3 healers 2 tanks and 5 dps.
and it looks like (if you can handle it) that 3 healers, 1 tank and 6 dps is the best way to go