I actually prefer Dominic Noah. Dominic is such a handsome name, as is Noah, but Noah is so much more popular. I like hearing Dominic for a change. Noah Dominic sounds a little bit better, but when you say the whole combo with your last name, even that could change.
Also, I don't love the nn Nick for Dominic. I like Dom, I think it's unique and cute. IF he's Nick, he's more likely to be the third Nick in his class than he is to be the third Dom. Honestly though, I would suggest waiting until he is born. When you are calling him names or referring to him, you'll naturally just shorten his name, and then that is the best nn for him!
P.S. I don't think Leah and Noah are too similar.