Definitely boost with the DK.
Get a lot of toons to 60th, on the four RAF accounts.

I don't see a Paladin in your list anywhere.
If you don't have one now, level one on the RAF accounts (or 4...).
If you do have one, that is under 60, boost it with the RAF stuff.
Or if its over 60, ignore it for now, but add it to a team when they catch up.

Go 4x Shammies on the RAF accounts.
Along with 4x several others.
Each team boosted to 60 by the DK.

I would suggest 1-2 toons who can heal if you want, on each of the RAF accounts.
And at least 1 toon who can tank on each.
Which opens a lot of doors down the road.
A melee team plays differently, DK + 4x Ret or a mixed team, or rogues/feral/enhancement, etc.

Both:
01-10 Quest.

Alliance
11-16 Deadmines
16-20 Stockades

Horde
11-20 Shadowfang Keep

Both:
20-45 Scarlet Monastery
45-55 Stratholme or Scholomance
55-60 Ramparts.