I didn't think a 10 planet map will spawn 9 nebulas, but yeah, even if it does you won't be able to get to them all. I turned on Partial Fog of War *just* so I could make sure that all 9 nebulas were accessible before the HW assaults.
Also, if you start with 8 planets on a 10 planet map, *all* the capturables will be on AI homeworlds. It might be better to do 7 homeworlds and reroll until there's a core world with a massive pile of capturables between you and the homeworlds. Snake map, obviously. That would let you get some Golems/Spirecraft before tackling an AI homeworld.