Polka dot plants fair poorly in most indoor setups in my experience, even in terrariums (although outside in the shade I have had them go from a little seedling up to two feet around in just a few months, and even bloom) That brings up another problem - if they do live, they will get big and grow quickly.
Many small vining plants would do well. Creeping fig has small leaves but does grow long so would need periodic pruning. A pothos or philodendron could be trained around the tank or kept pruned small. Fittionias stay short and if one were to get over 5 inches, just cut it back close to the base and it will recover. Small ferns take a while to grow and could be used for at least a year before outgrowing their suroundings.