mollycha...I assume from your comments that these will be permanent panels. In that case, I would suggest that you not attach them to the stucco, but through to the substrate. Recessing the panels will create a waterproofing nightmare...then you'll get blamed for water intrusion, mold, etc.!
Use a combination of pins and adhesive. The pins should be stainless steel, not rebar as the rebar will rust, create stains, and eventually fail. The pins should be installed fixed into the structure and the panels should essentially "hang" on the pins. The pins should be installed at an angle, typically greater than about 30 degrees upward and corresponding holes or slots drilled into the panels so the wind does not "suck" the panels off the walls.
Large stone panels are attached to buildings in a similar manner. If you have any questions, contact a stone facade company and they can likely give you the sources of their pins, etc.