with Elaine - No cactus.
I have a 20 gallon terrarium with a single bulb T5 light hanging over it. Mine is full of miniature orchids. I put a layer of gravel on the bottom, then a layer of sphagnum moss. The orchids are planted directly into the moss. Mostly Masdevallia and Dracula orchids. The two in pots are seedling Dendrobiums.
Whatever you decide on, use the terrarium to your advantage: plants that stay small and will enjoy the humidity. This terrarium has been planted for about a year now.
