That sounds perfect. <3 I like that. From what I know of Yusuke (slowly watching a walkthrough of Persona 5, since I don't have a way to play), I think Ran will like him and enjoy his company. Might occasionally be thrown off by him as others seem to be in canon, but I don't think that will bother her too much. Her in-canon childhood friend is much quirkier. XD

So those would be Ran's grandparents too on her mother's side. Sounds good! Could be another reason why Ran's mom has some issues (thinking she's prone to drinking and gambling a bit). She could either have gotten more pressure from her parents, or something happened with her brother or sister that caused them to not have the means to take care of a kid and that's affected her.