Is cactus soil all that is needed for a terrarium or are there layers of materials?

Usually you put a layer of pebbles in the bottom first. This is to help with drainage. Also a little aquarium charcoal to keep it from smelling sour. I am puzzled as to what plants you plan to use though. A closed terrarium would create humidity, not so good for growing cactuses in. Maybe you could leave the lid open a bit.

Any soil not too much water. Mine even grow in the snow. Texas Panhandle.