I've seen this theory before, and it makes no sense to me. I can't wrap my head around it.
Why do you think Serge is Magus?
Seriously. Why the hell does this theory even exist? You know that Serge and Kid hook up in the end, right? And that leaves a couple of possibilities depend on how you view the gecko ending of Cross, none of them good.
Worst case scenario, Magus would be dating his sister. Even worst case scenario, Magus would be dating his
niece. There are no words to express how fundamentally wrong this theory is with the above in mind.