Question, must know, was Kuronue one of the bandits before Youko Kurama betrayed Yomi, atau did he become an acquaintance of Kurama after wards?

Unfortunately, there are no facts at all about Kuronue in the anime atau manga. Both YYH film are not technically YYH canon. Togashi, the manga's creator, has mentioned in interviews that the komik jepang itself is the purest canon, and that even the anime strays from it considerably. If anda look at the movie's credits, I believe that the only influence Togashi had on the plot and menulis was... well, none at all. He created YYH, and the directors and writers of the film took his work and ran with it. Kuronue is not his creation and is not, therefore, YYH canon at all. No background about his existence, let alone at what time he appeared in relation to Yomi and Yoko's relationship, has ever been diberikan in any form.

But there can be denying that Kuronue exists. He's become a fan staple whether Togashi intended him atau not. I'd say anda can put Kuronue into whatever timeline of the series anda desire. If anyone has a problem with your choice, it's merely because they have a differing opinion, not because there is any factual evidence to back them up.