there are many plants you can use that will stay small. peperomia, dwarf philodendron, artillery fern, the list goes on and on. find a local garden center who carries terrarium plants. most are dwarf varieties that stay small. you can even use some aquatics if you like. glosso, staurogyne repens, some crypts, will work in a bog type set up. you won't need your soil below the water line to keep it wet, it will absorb water naturally if it is just touching the water surface. as far as substrate goes, i use an organic soil mix designed for hydroponics, covered in a layer of cocofiber. all my plants are nice and healthy.