Yes, firebricks are safe to use and a great way to take up space in an oversized firebox. Just make sure they are actually rated as firebricks but whatever your local hardware store has in stock is fine.
Many people just place them in or make slots for them to slide into which make them removable for cleaning. A high temp adhesive might work but check the instructions first to see if it will hold to metal. In the end, it depends on how permanent you want them.