To get your civilian fleet going, you need 1 colony with 1 million people.
Not necessarily. I started my colonies literally just by dropping a bit of infrastructure on Luna and Mars and Ganymede. The civilians built passenger liners and colony ships before I had my first colony ship built. It was slooooow going but there weren't any good minerals on those rocks so I was happy to let them build up the colonies at their own pace.
So all they really need is to see some infrastructure placed on an uninhabited world. Note that they will not colonize these worlds at the same rate. Luna got many more colonists than Mars which got more than Ganymede which got more than Mercury.
I'm sure that if you create a starting colony of a million people, that will enhance the trade opportunities and get them to grow their fleets faster. I went with a conventional start, so I had lots of money to subsidize them and lots of time to wait for them to grow.