Unfinished Business lets you find a new career for Reynald de Chatillion after the Fallen Paladins quest, but as far as I know, there isn't a mod that lets you actually recruit him.
As Senka points out, he isn't really evil. There are dialogue options in the vanilla game where he will walk away from the fight with Arnag, although I never seem to get those myself. I'm not sure that he's CG, but that's probably because I've never looked up his CRE in NI. I'll probably do that one of these days. (Checked the CRE in NI. He's CG.)
It's important to remember that a Fallen Paladin doesn't have to have an evil alignment. According to PnP rules, if a paladin knowingly commits a single chaotic act, he will stop gaining experience until he completes a quest of atonement. If he knowingly commits a single evil act, he will fall. It's possible for him to regain his paladinhood, but it involves a much bigger quest of atonement. Alignment is a summary of one's life, not something that is determined by a single deed, so at the moment of falling, a paladin is still lawful good. He may have a crisis of faith and become some other alignment after falling, however. So if Reynald committed an evil act and embraced a more chaotic system of beliefs, he could well be CG. Some paladins do become evil, though. Anarg certainly has.
If you're looking for a Fallen Paladin that is evil, there's one right here, Rylorn, that might suit your desires. He isn't romanceable, as far as I know, but he is definitely evil.
I wish someone woul develop Reynald as a joinable NPC. I love his story, and I'd like to see more done with him. But there are only so many hours in a day, and only s much time for modding.