The anime, the film and Angel Heart are based on Yuka Nogami's novelsIn her manga appearance Yuka had all but promised to write one or more novels about City Hunter, and, from what we've seen, she'll probably actually do it. The changes from the original story (the manga) to the anime are based on Yuka writing for teenaged girls and toning down some parts or completely cutting out others (see Union Teope being reduced to a much lesser threat and renamed Red Pegasus), and the film is based on that. Also, Angel Heart is a completely fictional work of Yuka based on City Hunter's actual story, and can be easily noticed by one particular: at the start of Angel Heart Ryo and Kaori were about to marry (something that Yuka told Ryo to hurry up and do quickly because her readers wanted their hero to marry his partner), but the actual Ryo has no legal person whatsoever and can't marry.
Ryo helped the Cat's Eye girls off-screen.Given he's often around at night and very good at both finding and remembering beautiful women, it's very likely Ryo saw them in action, and recognized them when he went at their cafe to court the gorgeous owners he had heard about. It's unlikely they hired him to help, but he may have given them some information for their heists and sold them explosives (the girls may have no care for collateral damage, but Ryo has a much better access to military-grade explosives).
The City Hunter is set in the same universeAnd Lee Yoon Sung was nicknamed City Hunter in Ryo's honour.