Question
I installed a 120 gallon pond last weekend, and spent this weekend laying natural rock and adding tropical plants. I have a filter/pump/waterfall set up at the moment.

I went to my local pond specialty shop and picked up some hardy water lilies and also some bog plants.

I was hoping to have 2-3 young koi(as the koi, and my wallet get bigger I will be enlarging the pond), but I am afaid they will eat the plants I have put into the pond. So, what are some plants I can get that wont be destroyed by the koi.

Also, I have entertained adding snails, or another type of bottom feeder to help with waste. Is this a good idea?

My Goal is to have a pond at about 700 gallons, so will 3 koi work well in the longrun?

Answer
To answer your last question first; three full-grown koi in a 700 gallon pond "may" be too much, but I personally wouldn't hesitate stocking at that rate.  Just make sure you're on top of the water chemistry.

Please don't get snails, you'll regret it.  Typically, it is illegal to sell creatures native to your state.  Therefore you usually end up buying snails that aren't used to your area's environment.  They will more than likely escape from the pond and get in the way of your yard work.  Use a solids-handling pump or siphon-vac vac to remove decade organic matter from the bottom of the pond.  On the other hand, your pond may attract a rout of local snails naturally.

Koi will eat just about any plant that you will find in a water garden.  If they can't reach the leaves, they will go for the roots.  It's up to you to find a place for the plants that your fish won't be able to get to.  Hopefully you have a soft liner and not a preform liner.  This would make it easier to position the plants safely.

Water lilies are possible if you use a special floating net that keeps them away from the leaves.  A couple of decent plant to put in your pond with koi are water hyacinth and variegated water lettuce.  They should be about $3.50/plant, they multiply by the hundreds, and the koi love to eat their roots as a snack.  The benefit is that it's not a $45.00 snack.  For the remainder of your plants, take care in potting them.  Place gravel as the top layer of the potter.  This will keep the "soil" chips from clouding the pond water as well as hopefully keep your koi from eating the roots of the plants.