Hello! I have a few options for you:
Gildea Dewberry is a sixth year like Crystal, though they're in different houses. Still, they'd likely know each other and share some of the same classes at the moment! Gigi also wants to work at the Ministry, so she'd be happy to be friendly with a fellow #careergirl. I could also see them bonding over art in a way. Gigi does metal and woodwork, but is not as familiar with "softer" arts. She'd definitely try to look over Crystal's shoulder why she worked or something like that. Also, as a muggleborn, Gigi isn't weird about blood status so ... friends?

@'Camilla Zabini' and Mireille could be friends? It would probably depend on how Mireille felt re: the Zabini's blood status scandal. Their time at Hogwarts would have overlapped slightly so they could know each other from there?