No, N400 doesn't supersede I-751. They are still separate cases so essentially you will have two cases to get approved for. However, N400 will never get approved until I751 has been approved. Including a letter for combo is just a courtesy letter; they don't have to act on it. If you still wish to, you can still add it now as unsolicited.
Nonetheless, you may still get a combo regardless especially if your field office is known for doing that. Even if don't get a combo, at least you gotten the N400 rolling and dodged the April fee increases. Good luck.