Yes, I think Jack and Luna should defo be friends! Harry <3s Luna.
And as far as your current chars. Obvs Remington would be a big connection for Jack. I think she would have been a year behind him in school? So maybe they occassionally socialized in the same circles.
Kit and I decided that Jack and Oliver were bros, so I'm sure he'll know Finley pretty well and will always be friendly toward her.
Maybe Lylliana and Sam can be friends? Sam super loves animals, so I can see her stopping in at the stables to see the horses and such.