Is it possible to use mobile minutes on a tablet Is it possible to make phone calls from a tablet and use minutes instead of data? 

The ultimate goal is to have a tablet (e.g. nexus 7) and a Bluetooth headset and make phone calls without using up all my data. 

I was looking into SIM cards to see if a different kind if card would produce the desired result, but I couldn't get a clear answer.
It depends on the tablet. Not all 3G tablets have the facility to make voice calls (just like a phone), but some do. Some have the hardware and OS support, and you just need to install a dialer app to use it; some don't even have the right hardware. The Nexus 7 in particular doesn't.

You should check with the seller or the manufacturer's website before you choose a device.