How about a Maranta, aka prayer plant? They stay pretty low, the leaves are fairly thin though but quite broad. They make a nice cover for the lower parts of a viv but not great as stand alone perches.

There are trailing kinds of Peperomia obtusifolia ​that don't grow very tall, the leaves aren't enormous but are very fleshy and sturdy. The upright versions might work for you as well- it takes a few months to hit your 18" mark, then you can just cut the top 5" or so off and stick it back in the ground.