Chickens is chickens. They'll all get along, no matter what breed you add. The only caveats to this are that fancy feathered birds like Polish and Silkies don't always do well in mixed flocks.
If you just care about egg-laying ability, you could add some Black Sex Links--pretty birds, lay like crazy. Easter Eggers (hatchery Ameraucanas) tend to be good layers and you get fun green eggs, but they may be flighty. I have not had luck with Delawares. Have you considered Barred or Buff Rocks? They might be my favorite breed right now, after my Red Sex Links.
