Is it OK to use my Nook charger to charge my ipadmini?

As long the power adapter provides 5 volts DC at enough amperage, 5 watts for iPhone and 12 watts for iPad (not sure about iPad mini), the device will charge. There is no intelligence in the power connection. The intelligence is in the cable where it connects to the device. As long it is a quality source there will be no problems. I use many different types of power sources for my charging needs. I just need to use an Apple cable.

As long it provides power through a USB port it will work.

I think it would be a better idea to use the charger that came with your iPad....