If you aren't including Everett or Tacoma I don't see why Federal Way counts. If anything it's a suburb of Tacoma (and parts of the city are further away from Seattle than Tacoma is). I'd argue the same for Auburn using your current definition.
What's your point? There is continuous development between the cities and they have close economic ties. Bellevue, Tacoma, and Everett (usually) are all considered part of the Seattle metro area.