Thank you both very much. Here's the update: I've set up a quarantine terrarium with some pillbugs, baby millipedes, moss, and lichen. Everyone seems to be doing okay so far, even the moss, which I wasn't sure would take.
Legomaniac, that pipe trick sounds really cool. I've never heard of that before. I'm not sure where to look for the plants in town, but if I can't get them I'll try Hydrophyte.
Lauraleelbp, weed fabric sounds like a good idea, but I have no idea if the pillbugs and little burrowing millipedes will eat it though. Have you had any problems with that? I did consider creeping fig, but I wasn't sure if the tank would be too wet for it. I think you're supposed to let the soil dry out between waterings for that, right?