shadowfax: Welcome to ATP! That's a beautiful tank you are using for a terrarium, I look forward to seeing photos when it's all complete!
Miniature Sinningia's are perfect choices for terrarium culture and another great little miniature Gesneriad to consider is
Chirita (Metapetrocosmea tamiana) I grew a few of them for about ten years, some in terrariums and others in little pots. Some Chirita's get really large but the tamiana is a miniature that stays small and blooms it's little head off.
Rex Begonnia is another that I've grown in terrariums with great success: