Frog would be a Peridot and their gem would be on the back of their neck. They'd vibe with Canon Peridot and they'd just infodump with each other for hours and it'd be great.
Dick would be a Sapphire and his gem would replace one of his eyes. (He and Juniper would probably fit better if their roles were switched, but the colors just work too well.) He'd probably be a more animated and easily excitable Sapphire, making him fairly different from Canon Sapphire, but they'd have some very interesting conversations
Juniper would be a Ruby and his gem would be on his forehead. He'd be more of a Navy-ish Ruby and not want to punch everything in sight, but he's good just vibing with his fellow gems.
Shiloh would be an Amethyst and her gem would be on her chest. She would 100% hang out with Canon Amethyst, as they share a lot of the same vibes.
(and for the next two we shall ignore the fact that they are villains)
Vanessa would be a Fire Opal and her gem would be on her forehead. She and Lapis have a similar sense of humor and I'd love to see what kind of conversations that they'd have.
Alex would be a Citrine and his gem would be on the inside of his left hand. He'd do his best to ignore everyone else and just hang out on his own, but he'd probably end up being thrust into conversations and he'd end up enjoying himself.