What are the rules regarding which deities can be used as half-blood parents?

Scrolling through the Character Tab I notice a couple of demigods who are the children of some of Greece's maiden goddesses (swore a vow of chastity) and a couple that are the children of primordial deities that typically didn't interact with the mortal plane beyond the aspect they embodied in nature. I haven't read all of the bios so I don't know if there were special circumstances regarding those, I'm just wondering are we allowed to choose basically any celestial being from Greek mythology or should which deities that could or can't, would or wouldn't be limited by the Greek myths?