Personally, I don't have any ideas for what Rubi could do, so can't help you there
As for Fler and Beryl's new friends, personally I quite like them. Seeing the girls interact with other characters helps develop their own character (like, back on the first game, I never imagined Fler hanging out with the geeks and enjoying it, but having seen it in the second game I liked it)
The only thing I worry about is how it'll affect their romantic involvement with Niko. Right now, only Beryl has shown signs that maybe she likes this other boy, but it could become something for all the girls eventually. I think it can work as long as the player is given the choice (have her go out with the other boy, keep her to yourself, do both, and so on)
But then again, this is your game, so the choice is up to you