Where did you hear about this? I've never heard anything about any clone troopers in Rogue One, much less Captain Rex being a villain in it. Google searches for "captain rex in rogue one" and "clone trooper in rogue one" bring up nothing, while Dave Filoni (showrunner for The Clone Wars and Rebels) has said he likes to think (though won't confirm) that Rex is the old bearded Rebel soldier on Endor in RotJ: http://www.flickeringmyth.com/2016/04/star-wars-rebels-dave-filoni-thinks-captain-rex-appears-in-return-of-the-jedi/

I'm just curious where you heard/saw this, cause this is the first I've heard anything like this despite checking Star Wars news and leak sources far too frequently for my own good. (As far as canon viewing for Captain Rex's current situation goes, episodes 1-4 of TCW season 6 and the episodes "The Lost Commanders" and "Relics of the Old Republic" of Rebels explain how Rex evaded Order 66 and ended up joining the Rebels; the TCW episodes aren't strictly necessary, since the takeaway is summed up in the Rebels two-parter, but that particular arc is one of TCW's best, so it's worth watching anyway.)