I think this decision would be easier to make if we knew their personalities and role in the story.
Mantarian - Male
Celestgules - I think the male looks cooler but since you said they are supposed to take on human shape then I would choose female, since the male doesn’t really look human at all,
Daemoir - Male, since it would be cool to have a rom between light and dark beings.
Silvabor - Famale
Oracoth - Female