Oh boy, so many questions.
"Can a PC god have a NPC Miko?''
Excellent question. I am tempted to say ''No, but there will be exceptions, if you REALLY can't get anyone to become your Miko.'' There's also the other possibility which is presented in the next question. But anyway, a god is by far the "highest VIP class" in the game, and creating a god is not possible until you've become more than familiar with the game, so there's no need to worry about that now.
''Can a youkai be a Miko?''
Sorry, but that's nowhere near possible. Hardworking Mikos who form a strong bond with gods cause their shrine to become a real youkai repellent, so it's physically impossible. What IS possible, however, is that a youkai becomes the servant or close friend of a god and helps him out and serves him. It's a very touchy relationship because youkai's faith don't give any faith points to the gods, so miracles have to be out of sheer generosity or rewards. Gods by themselves also usually hunt youkais and are meant to be the ''higher ones'' for humans only.
"Byakuren and Miko in the same sentence"
This kinda made me cringe. Byakuren is extremely far from being a Miko. The reason she was sealed is because she, to put it bluntly, sold her soul to the devil by using the art of dark magic to make herself immortal, which is by itself an insanely huge act of heresy against life itself. She does have a few "light spells", but they're more or less linked to her magician side fused with her beliefs, as she use light elemental magic (rather than holy magic). While she does pray Bishamonten, he is far from being a simple god; He's the very embodiment of war and battle, a concept that simply CANNOT be erased. Furthermore, even her life as a buddhist monk is more of an encouragement for youkaiXhuman lifestyles. In the end, Byakuren is a magician due to her dark magic and a monk due to her beliefs.
Besides, a Miko is the very embodiment of the shinto religion, while Byakuren is a Buddhist; Her being a Miko would be pure heresy.