Yes, like anything, you do need to QT Ghosties. I didn't because they live on their own and came with each other, but if they are going to be with a Betta then yes.
You can just keep them in a little bowl for a week or so. Monitor their health.
You don't need a filter, and you don't need to do a water change while in QT.
Ghosties can thrive in any type of substrate. So yes, pebbles are fine.
*2 Bettas (for now RIP Ryu) *2 Goldfish *9 Guppies *1 Gourami *2 Chinchillas *2 Budgies *2 Dwarf Frogs *1 Dog *1 Cat *2 Hermit Crabs *2 3gs *1 10g *1 Pond *2 2gs