Bro..some general info based on ponds I had.
Bottom drain, I recommend use 4 inch pipe.
The rest of the pipe, I use 2 inches, including output pipe from pump.
For the connecting to waterfall, you can reduce the vertical pipe to 1.5inch if needed.
For me, I use 2 inch pipe all the way, including the water fall. The vertical pipe length is about 5ft. No issue on water pressure loss.
I recommend use Tsunami as it is most reliable so far for me.
I would not recommend aerated bottom drain for your case, as the air bubble will affect your viewing.
You need to have a calm water surface for viewing pleasure.
For air bubbles in main pond, you can put at the side of the pond wall. In your case, I suggest on the left wall, so that it will help in "pushing" the water towards the surface skimmer. surface skimmer is importance for koi pond.
for your standing drain pipe, they should not be connected in single discharge. Each should have their own discharge.
Are you in Singapore?
