You're better off using flex pvc, for the reasons described in your thread. But it depends on how big your system is. A 30 gal tank w/sump system dosen't need to move 1000 gals/hr so it can be done with clear hose from the hardware store using barbed fittings and clamps. Where, say, a 120 needs to move more water, will need bigger pipes, the ridgid pvc or flex pvc would be a better choice. Try to picture where you want to go with the water. From here to here. It's that simple. The hard part is realizing whats available to you to accomplish this, (fittings, valves, connectors). As stated above, using flex pipe you can avoid using 90 degree bends(elbows) which create restriction of flow (backpressure) against the pump, which will slow down the water flow. but in not using fittings to make the bend, you have to make wider loops or bends to make the connection so you don't kink the pipe. Instead of using a 90 degree fitting, use (2) 45 degree fittings. The use of unions allows you to easily disconnect and remove a component (pump, over flo). There are a lot of different fittings and connectors. Yes there is an "art" to this, that just makes it look good, while being done correctly makes it function the way it should. I think the best way to understand it is to see other persons systems and how they did it. Although I have worked with pipes and fittings for 35 years, initially getting in this hobby, I had a hard time understanding why the water flowed from this point to that point, and then back to the tank. But going to reef club meetings, I was exposed to several different setups and realized there is no one way to do this. As as I progressed in the hobby, I made improvements where I learned they were needed. Hope this helps.
