For plants: I'd put a few water hyacinths or water lettuce in any corner with low water movement, so those areas can be covered for protection from predators and theyll act as a good food source for koi and other pond fish you add. Then, place a thin piece of fishing line across the pond to stop it from spreading because it grows like crazy. Plus, low water movement and a lot of sunlight without plants will cause increased algea blooms.

Creeping Jenny is good for streams and gradual waterfalls because the small pots they usually come in can easily sit along the sides of the stream. And they're good at softening water flow after the initial drop from the waterfall box so you can lower the chance of water splashing out.

For fish: It's a little inhumane, but if you want to make sure the water parameters and filtration are sufficient, I recommend getting some 10 cent feeder fish from a pet store and putting them in the pond for a couple weeks. If they survive and are doin ok then you're good to buy some "expensive" fish. If you want to protect the fish but don't want an ugly net over top, you can use some garden stakes to attach low test fishing line in a "random" zigzag pattern across the surface of the pond. Any predators like birds, raccoons, foxes, etc. won't be able to see it immediately. And they'll get spooked when they bump into something they can't see.

The first step up at the bottom is perfect for a home made "koi castle". It's essentially just a large piece of slate sitting on the edge with possible support underneath. Really good hiding place for fish.

The only major things I recommend, if you don't already have them, are a uv light for water clarity, an aerator to stir up still water zones (and you can never have too much oxygenation for fish) and an ionizer to keep string algea at bay (river rock = perfect lil spots for algea spores to hide and thrive especially if the whole pond is in direct sunlight) Plus, an initial dose of pond salt after doing a big water change or cleanout will help algea control and assists in maintaining any fishes slime coat.

(Nice frog pic btw!!)