I would think a solo Enhancement Shammy would add a lot to a melee team.
Not sure if multiple shammies is the best option.

The more classes you add, the harder is becomes to get optimal dps from each.
However, DK tank, Enhancement Shammy and two Ret Pallies as a team.
And someone else playing the healer, or if you are... another class too.
It's a few things to manage.