a 6mm axle on a featherweight will get bent VERY easily. Take a hammer and with your thread in a vice smack it. It will bend. If you are not happy to do this on an axle in a feather then it isn't suitable for the arena. Ideally a featherweight should be able to fall from a good 6ft onto a hard surface on any part of the machine and survive undamaged.

O and the blue wheels have a bore around the 12mm mark (can't remember exactly). Save yourself some time, buy some drills, take out the motors and gearboxes and use them with the robochallenge wheels with the nut insert.