He did not "farm" the rinnegans. It was the chakra receivers that were placed in the EDO jin's that allowed them to take on the controllers eyes. The same way the 6 paths of pain all had the rinnegan that Nagato did, the jin's had the same eyes Tobi had. Do you think Nagato was able to "farm" out rinnegans? Considering he often changed the bodies of the 6 paths he would needed a large supply of rinnegan eyes.
they all had chakra rods... Naruto went inside Sangoku (4 tails) for example to bash one out from within his body.
I'd like to add that normally the six paths have two Rinnegan. But Obito only had one. So his Six paths also had one.
They didn't have a sharingan, because he had one. This only works for Rinnegan. Obito put a sharingan in everyone's right eye. Otherwise they would have one normal eye and one Rinnegan.
He would never replace a Rinnegan with a normal Sharingan.