RDWC help please

So I'm building a RDWC undercurrent system. And I'm starting to get confused on the pipe sizes etc etc. So let me explain

For the pipe sizes. Can my return pipes and flow pipe be the same size? Or does the flow pipe have to be bigger than the return pipe? I have ordered all 19mm(0.7inch) fittings and pipes. And bought a 1100lph but going to get a 2000lph one instead.

Second question is. I am using 3 buckets one for a resivour. I want it to be in triangle. With the return pipe connect to a tee connecting to both the plant buckets. In that middle return line. I was to connect another tee with a inline tap on it to drain the system. And will have a inline filter connect to return line.

So for the return pipe. Is it better for undercurrent of should I make it waterfall? So instead of connecting the return pipe to bottom should I waterfall it instead or doesn't it matter?

I have ordered everything waiting for it to come. But I'm worried that my pipe is too small and won't get a proper flow. Any help or advice will be so much appreciated. And should I put the return pipe from resivour on top of buckets or underneath.

Pipe size and fittings are all 19mm(0.7) return and flow are same size. Is this okay?

 

I posted a pic but here you go. I'm having hard times trying to find a 2 inch or 3 inch tophat grommet to connect the pipes. All I can find is 19mm ones :/ what else else can I use to connect the pvc to the buckets for drain?