Could Ryuma be stronger than Roger and Newgate? Possibly. After all, Ryuma was the protagonist of a totally different story. However, he has now been incorporated into the world and story of One Piece. And in this story and world, pirates, and to a lesser extent marines, take precedence.
Oda has placed more importance and reverence towards Roger and Newgate, and not just for their notoriety, but their strength as well. This isn't factual proof they are stronger than Ryuma, but the reverse can be applied that there is no factual evidence Ryuma was above them.
If I'm going to give my honest thoughts, I still feel Roger and Newgate are stronger. They are simply more important figures within the story, and they are benchmarks for the protagonist of the story to overcome.