how many and/or what type of resource asteroids spawn around planets
unfortunately not
and how to set neutral colonies
There might be a way to do that, using player settings and templates, but I haven't tinkered with it. That said, you can easily create starting colonies for players.
the big drop down list with stuff only has 1 planetary bonus: Crystal miners
I've never understood why that's there and the others aren't. I suspect there might be a way to add the others, but not in galaxy forge. If it's possible, you'd have to modify the map by hand (ie, open it and type what you want with notepad).