I think that James will have known Greta from school. I think she'd have been a year ahead of him? I figure they'd get along okay though he wouldn't approve of her not siding with the Order. :P He likes the stubborn witty ones though so, they should definitely interact!
Antonin is amused with her job and is morbidly curious what she would come up with to say about him if he died. I don't know if they'd have a reason to interact but we can come up with something eventually?