If you make the maps in Galaxy Forge, I believe you can set a team index to the starting locations, and that way teams will start where they're supposed to

I haven't done much map making, but I'm pretty sure that's how it works.
If you use in-game map maker, you can generically set number of players per star, and then you can spread out however many players across however many stars as you wish, but the rest will be random as far as which system everyone spawns in.