Those coreless motors are designed for small quadcopters and RC helicopter tail props - you'll find
props that fit the shaft easily - gear trains even, if you look for RC helicopter parts. Not my
first choice for robotics - micrometal motors are available in several voltages and many reduction
ratios and have metal gears, and are good value on eBay, for instance:
http://www.ebay.co.uk/itm/1Pcs-6V-100RPM-Micro-Torque-Gear-Box-Motor-Hot-Sale-/231466738662?hash=item35e47e23e6:g:r8wAAOSw1vlUyzNE
But first work out what speed and torque you actually need before choosing a motor, there's little need
for guesswork.