This is a really cool mod, awesome concept!
My suggestion would be to lower the cost to around 300,000 initially, but then have to pay more credits to expand the station's max size every time the population reaches 100% for the current size, or else have the station remain the same size. I'm not sure how easy that would be to code, but I think it would make it a little more accessible without making it overpowered and lore breaking, since you'd need to invest more resources to grow it over time. It would also keep said station from growing from a few dozen people to greater than the current population of earth all on it's own, which would be a little bit ridiculous.
I've seen a lot of discussion on the balance of this mod and while my experience with the game is limited there might be a way to deliver a scaling cost to these stations, making them cheap to initially build but require a serious investment to get them to the same level as vanilla stations.
Assuming you want to balance the cost, hazard rating and maximum size, give stations a set of unique conditions. One could increase the hazard rating, another to limit their maximum size, then create unique structures akin to the "Military Police Headquarters" from your terraforming mod that changes the condition (with one increasing maximum size and another reducing the hazard rating). If done in simple increments, say starting at 200 hazard rating, then going to 150 then to 100, and the size limit going from say 4 to 7 to 10 (best would be to make these configurable like you did with the other modifiers, so players can balance them, or have the option to disable them completely); Then you could meet most peoples desires, by giving these buildings a good cost in terms of credits and resources it allows you to make them both easy to set up, but require a significant investment to get them to the scale of a planet.
Physically speaking for the size it would make sense that only the initial infrastructure is shipped in the beginning. When comparing them to existing projects (the ISS or any other proposed space station) it's generally a better idea to build enough infrastructure for the current project and the immediate future. As for the hazard rating, you could take it as the station being fitted with the necessary equipment to survive the particulars of that orbit (maybe the local star has a tendency to project small solar flares that wasn't taken into account on the design of the station or the local magnetosphere is much stronger than expected).