Q: Could a Shinigami (Bleach) ever eat a Devil Fruit?
A: In theory, yes. But I'm going to be altering the way the spirit realm will interact with the rest of the world.

Q: Could someone that uses Spirit Power (Yuyu Hakusho) make use of Kiddo?
A: Sure, why not. As of now, can't think of a reason why one couldn't.

Q: Will we only get to decide on one, uhm, anime, for our character's to gain powers from, or will there be some mixing and matching, even if only through story?
A: At the start, you only have one. You'll get more later on.