Also in regards to the 'sun pokemon' being weak to fire...lets look at the dex entry/description:
"Since ancient times, Solgaleo has been honored as an emissary of the sun. It is referred to with reverence as “the beast that devours the sun.” Solgaleo's body holds a vast amount of energy, and it shines with light when it's active. It has a flowing mane with a remarkable resemblance to the sun."
They're referencing the symbol for purifying metals from alchemy, which is a green lion devouring the sun. That's why he's steel/psychic
Hardly a game breaker, I applaud them for thinking outside the box.
Edit: Lunala has it's origins in alchemy too, mainly the belief that the moon, being a source of moisture, releases aqua merifica, a liquid believed to extract souls from the body and give bodies life and soul. So there's your ghost/psychic reasoning.