Looks like the ones you posted pics of come with a mounting bracket that attaches to a luggage rack. You wouldn't want to use them on a OEM B12 luggage rack for a 2G as they're only rated to carry ~5lbs. There are racks out there that can carry more. Look for one that mounts in more locations than just the grab rail.
If this is for a 1G, I don't know as much about their luggage rack details.
As for using them as side cases, I guess you could, but you'd probably need to figure out your own mounting.