1. Yes. Up to 20 players, any mix of human or AI.
2. sorta. Not initially, but when you've gone through a game you can save and re-use your empire, including the ship designs. Note: this doesn't work as well as it sounds, because you won't be able to build most of them due to too-low tech levels.
3. Yep. Or make a completely new race for that matter. Just have them make their race, then save it and email it to the game master.
4.
5. Per SYSTEM. Note that sector in SE4 terms is an individual square. Keep that in mind or some abilities will confuses the hell out of you.
6.