I don't really know what the deal is with Scott and Melanie.
Holly seems like the classic "girl you have fun with" and Melanie seems more like the "woman you'd settle down with".
Scott didn't have anyone paying much attention to him other than Toni before Holly arrived and I think it was pretty obvious he didn't see Toni in a romantic light.
He now has two women interested in him.
Even though Melanie told Holly she wasn't trying to muscle in, in a game where you have to pair up or ship out, you need someone who will "have your back" as they say.