Cowell initially chose not to advance Amaro to the live shows following her performance at Judges' Houses, but then realised that he had made a mistake and asked her to be the fifth member of his girls group for the live shows.
Reid, who is mentoring the boys, told reporters on a conference call: "I was very happy that Simon did go back and rethink Melanie because I thought she was really fantastic in the auditions. However it works out, I think so far it's working out for the better."
When asked if he agreed with Cowell's choices, which also include Tiah Tolliver, Simone Battle, Drew Ryniewicz and Rachel Crow, the record executive admitted that he might have chosen differently based on their different tastes.
"I have a very different taste than Simon so I may have made some different choices, but Simon is incredible and the most experienced guy at what we're doing here on X Factor. I'm here to learn and watch and see what he does."
Reid was also asked whether he has spoken to his friend Justin Bieber about Bieber fan Ryniewicz, who wowed at her audition with a twist on his hit track 'Baby'.
"I haven't spoken to Justin about Drew at all," he said. "I think she's amazing and really a contender here, a real competitive talent.
"Justin is a good sport and takes it all in the right spirit. He doesn't take it seriously when someone says they want to be better than him or beat him. He has a level head and he takes it all in fun."
When asked for his thoughts on Nicole Scherzinger's decision to take Dexter Haygood to the live shows in her Over 30s group, he said: "I like Dexter a lot and I respect Nicole's decision to put Dexter through. He is clearly an artist and performer and in this case a contestant who has gone through some tough times in his life.
"I think it was [about] believing in someone and giving them an opportunity to rebound and get their life on track. He's clearly talented and a showman but I don't think it was a decision based on spectacle, I think it was opening a door and giving someone an opportunity to straighten up and that I support 100%."
Reid added that although he is happy with the talent in his own category, there are contestants in the other three categories that he would have loved to mentor.
"I will say there are people in other categories who I love equally and some of them I wish I had," he said.
"I won't tell you [who] but you'll see as the show goes on! It will be pretty obvious because I don't have a poker face! If I like something I react to it and my reaction will tell the story."
X Factor airs its first live show on Tuesday, October 25 at 8/7c on Fox
Watch Melanie Amaro's first audition below: