[kerensky]

...you could start them as unattended empires and make the centrally located old-homeworld empire fill out all of space and then 'gift' the various pie-slices to the unattended little empires, which then may or may not get to keep their homeworlds.
You need not fill all the facility slots on all the colonial worlds, leave that for later development.
The old-homeworld should be absolutly badass: many, many Large Plats, Fighters, Troops, defensive Star Bases.

This idea sounds like it would be easier than trying to create several parralell empires. Never done a PBW but I would be willing to try.