Not generally if you plan on planting them in the media. As mounts is a better idea.
Depends on how much you're willing to do to keep the 'filler' in check. Many will very quickly try to take over the entire tank. (Witness Mass's ivy -- though I've always found ivies to be major spider mite magnets ....).
If you want rampant growth and don't mind "weeding" regularly:
If you want slower growers:
some types of orchids
Really depends on what you want and how much money you consider reasonable.
If you're talkin CP terrarium, Utrics are great & they flower. Also you could try LFS moss for any terrarium. I have some miniature ferns that i use in my terrariums, too, that send out runners but are slow growing. there are also some tropical miniature begonias that love terrarium conditions. A lot depends on how damp/soggy your terrarium's conditions are going to be & what kind of planting medium you're using.
Just to give a lil info all of my plants will be in pots. I plan on filling the terrarium to the rim of the pots with LFS. I just want companion plants and space fillers to give a more natural look. Thanks again for all of the suggestions and I will search all of the plants and see what I like. Thanks!