Shipping Lines always form at the capital. There is no minimum pop or spaceport requirement but there is a requirement for you to have two populated worlds of any size (even 10,000 colonists is enough). The 10m pop requirement is for civilian mining colonies. They will only appear in a system with a 10m+ population.
Spaceports are now purely for improving loading times. That can make a significant difference over time to the amount of trade goods, infrastructure or colonists you can move, especially to nearby planets/systems.
Steve