Nyx has a son but I don't see Choas taken. I don't think Saarebas would mind Dom having a stepsibling. Everyone is sort of related anyway

No character limit as long as all characters are equally involved

For a new goddess you would have to create a WHOLE history for. Im fine with similar powers if you can make it different enough

I don't know. I think whether the child enjoys fighting depends on who they take after. Sparing is aloud within reason I would prefer you wait for fighting class.