What are some good beginner plants for a cactus/succulent terrarium?
Unfortunately, truth be told, there really aren't any for a closed terr. This question comes up every so often -- not surprising as there are some neat cacti and succulents out there. That being the case, rather than simply omit any reference to desert terr, it seemed wise to address this issue.
Cacti as a whole do not make good terr inhabitants. The main problem is moisture. Most cacti and many succulents come from extremely arid environments. The terrarium is just the opposite - the whole idea is to maintain humidity. Faced with the higher humidity & stagnant air, cacti and many succulents do very poorly and soon suffer from fungal/bacterial problems. Light is another problem. Most require very high light levels. But in the close confines of a terr, this results in high temps that would "cook" them.
Now an open terrarium would be a different situation. It would likely be very challenging but might be possible. As people experiment with plants for such a terr, I hope they will let me know as to their successes and failures.
Some possible candidate plants to experiemnt with if one does decide to attempt an open terr/desertarium might be:
sago palms (while they are small, of course)