In my map map6 I created a new system #254 named Central (see really big games). I specified the system type as Nexus, which is not a recognized system type. In 90 passes I added 4875 starting planets using a 10 planet start in the game for twenty races. It never put any new planets in Central. So you can be sure that a race will never start in a system if it has a nonstandard system type.