In the interest of making this more elegantly direct and making Migrant/Exodus fleets possible, they should just make a rare tech or line of tech that make it possible to have any tile (Building and pop included) be converted into a space ship. That'd allow you to have certain base functionality in a migrant fleet situation, or if you wanted a mobile capital, mobile space station, etc, you could do that too. That way the Migrant/Exodus fleet could include a clutch of mining ships, power plant ships, etc. as the civilian core of its fleet and still have a semblance of the functionality of a core sector while keeping the flavor and limitations of a nomad empire intact (You'd probably never be able to manage more than a small planet's worth of such a nomadic fleet, which would match sci-fi precedence).