What is the name of the long leafed plants?
Do you mean the variegated one at the top of the waterfall? That's a type of grass - not sure what variety. They grow really big!
And speaking of big - I'd rethink your plan to get one koi that "doesn't grow real big". There's literally no way of knowing which koi would end up a 36 inches, unless you're buying something really expensive and are absolutely certain of the bloodline. Your garden variety koi can do just about anything growth wise - we have some that stayed small-ish and some that are monsters - all purchased at the same time from the same place.