As I understand it so far, the Death Knight starting area is instanced somehow - meaning we might not be able to bring in a higher toon to powerlevel them.

So my current plan is to make four Death Knights (all dps to level with) on my four accounts, and run them as a team through that starting area. They ought to level pretty quickly.

Edit: As far as using one as my main tank - when the DK matches my Paladin, I'll then figure out which I like tanking better on. (E.g. Which is easiest to control mobs in instances with while multiboxing them.)