Today asked a Vag technician I know. G12/G12+ and the red G13 will all work fine in a leon (and can be mixed). All three can be obtained in a 50/50 version and pre-mixed ready to use.
If not too much water can be used to top up (if the right coolant is not available). He strongly advices to use distillated or de mineralized water (which is cheap) and only take tap water if it is very low in minerals.
He told me it is normal for new motors to "use" a bit of coolant since small amount of air can still be in te system when the car leaves the factory and will dissapear after some time.
After that no more coolant should be used in a healthy engine.