The people in the Vivariums thread could probably help you better than I could, but don't underestimate the ability of pothos to grow huge as long as its pot is big enough. My two dumpies haven't been able to beat it down and they've tried a lot.

Also, not knowing what's available in your area, you might want to go to a nursery or somewhere and take a look at the houseplants section. Anything that's good in low light and moist soil would probably work. I had some bamboo for a while, but I had to keep cutting it back. I've also found really beautiful Peacock fern that would work for a mossy groundcover, but I had trouble keeping it alive. I've seen vivariums with African Violets in some people's tanks, those are nice.