So, in case anyone is wondering, the additional interview letter they sent was Form G-56. (Didn't see the form name on bottom left until the interview date). They send this letter if they need additional information or evidence or call in interview.
Sometimes, people take lawyer with them if they get Form G-56, but we didn't take one.
IO was super nice and never asked for any additional evidence. IO just asked questions about our marriage and some questions from both of us. N-400 was approved on same day. I-751 was approved the next day.
Thank you everyone for the help.