The pressure required to drive the planter will be essentially the same whether it is driven by one motor driving the entire transmission, or by twelve motors (of the same size) in series driving it two rows at a time. 

You need to use a motor big enough to take about 10 gpm in order to get enough power to run it smoothly.  Going too small will increase your operating pressure and in my opinion cause more problems then it solves.   I think the ones you link are about 1/5 of the displacement of what I used.  You can of course vary that some by changing sprocket combinations, however I think I had about as much reduction as I could get in a single part chain drive, so in order to use a smaller motor and maintain oil flow, you would have to go to a jackshaft in the drive to get more reduction.   I actually did not build the planter, I only added three motors to it in order to get more sections.  I used motors to match what was already installed on the existing sections, as set up by Big John and the dealer. 

The item numbers in my old links appear to be good, but the links have changed probably due to site updates, or else they got mad about being hotlinked.  I can't believe someplace trying to sell stuff would be upset about free advertising, but who knows.