Hmmmm. A cactus/succulent that can live on lower light, without spines, and (though you didn't mention this as a requirement) stays small enough to remain in the terrarium...
Jade, aloe, sansevieria, and some euphorbia spp. would probably do. Stay away from the variegated/colored forms as they need more light. Also, as you stated, plant them shallow in a loose mix as succulents are prone to rot in lower light.
I'm not sure whether the lizards like to eat plants, but if there's any chance that they do, I'd ELIMINATE euphorbias from the list as they have a milky sap that is irritating and/or deadly to most animals.
I'll try thinking of some more.