Yeah. Like I said it depends on if you already have some gear. It sounds like you plenty of resto gear so it's personal preference as to which you want to gear as a Dps caster. Ele or Bal. As long as your DK is defense capped you should be fine. It will be a little ruff at first but you will find that having your DK tank will add nicely to your overall DPS. Also keeping him an unholy tank will also give your caster heavy group a nice spell power boost as well.

Another thing to think about, If you keep all your shamans elemental you may lose out on a little crit but it will make DPS'ing nice and simple since all 3 have the same spells. You don't have to watch for procs like you would if you made a boomkin. That gives you more room to think about DK rotations and healing.