So I've been contemplating writing a fic that takes place about ten years or so before RSE's events. Because of this, it wouldn't make sense for some Gym Leaders to actually be Gym Leaders, like Norman who moves from Johto right before RSE takes place while a handful of others (Roxanne, Flannery, Brawly) would just be too young to be Gym Leaders at this point. The nature of the fic I plan to write will include a few Gym battles, so I needed to replace a few Gym Leaders.
I have come up with a few replacements, like Flannery's Grandfather as the Gym Leader for Lavaridge and possibly Palace Maven Spenser, because his staff is reminiscent of Kyogre and he apparently touched the Blue Orb according to the Manga, but I can't really place him into a town. Other than that, I'm drawing blanks.
Would it be okay to make up replacement Gym Leaders or would it be better to make up new towns and new Gym Leaders? If they were 'replacements' then would they need to have the same typing (e.g. Rustboro's Gym Leader would be a Rock-type trainer)?