First - I have never done hydro, so take anything I say with that in mind, but I have done a ton of reading recently.
Here are my takes on this.
1) Its not that critical.
Almost any system will work just fine and produce very good results - IF - you do ALL the other things correctly - proper nute levels, proper PH, proper lights, proper temps, etc.
People get good results with hempy buckets, water farms, DWC, RDWC, NFT, aeroponics, E/F, etc etc. Those techniques range from zero circulation to lots of circulation, and from lots of water to almost zero water, and from lots of extra aeration to zero extra aeration.
2) The only time pipe sizes matter when your talking intake vrs out flow is when your depending on gravity to return the water to the holding tank. In that case, you need the outflow to be at least as fast as the inflow from the pump. Its not a linear relationship though, so you cant just say use twice the size of drain. A hi power pump will require a much larger drain size than a low powered pump. The length of the return and any elbows,, etc will also effect the rate of flow.
Darn - gotta run - more later....