Some theories are though that Guile is Magus
It's not a theory. It is partially the truth. At first, Guile would be Magus. However, with so many characters the developers thought that they wouldn't be able to develop his personality. The Guile=Magus idea was abandoned, so Guile is just a wizard and Magus is not in Chrono Cross, although he might have done some things behind the scenes.
Serge being Magus (or relative) is not so crazy... Actually, there's no clue that he isn't. Lucca mentions that Janus might be next to Kid when she reads the letter. There's also the blue hair that only Zealians have (and they dye it, unless Magus used magic to make this colour permanent, something that might have been passed down to Serge - in case they have relationship).
There's also the fact that Magus, not Serge was the one who actually wanted to save Schala. Schala creates Kid to save Serge. Why couldn't Magus create Serge to save Schala (after being saved by Kid)? He has magic stronger than Schala's by the way...