400mu is the population. It will stay at 400mu if your homeworld's terraform level is 80% unless you do a few skills in the science tree, sorry I don't remember the exact names but one lets population increase per rank and then the one that lets you build terraformers.

Yes the 4 planets in your starting sector with your home world are already outposted just make a harvesting route, third button when you select a harvestor, and select the planet you want to harvest as the source and your homeworld as the destination.

When you create an outpost ship you want to send it to another sector, as the ones in your starting area are already under your control.