If my regular propane tank for the house runs out, can I use a BBQ tank as a temporary fix?
This generally won't work all that well. While both large and small tanks contain propane, the large tank can deliver more propane to appliances than the small one can. 

Propane is supplied as a liquid under pressure. The pressure in a propane tank is pretty much an indication of its temperature, not the amount of propane in it. When propane is drawn off by an appliance, the propane in the tank boils to make more gas - and that boiling lowers the temperature of the tank. A small tank, especially a small tank in winter, can become so cold that it cannot effectively supply gas.

This PDF (which unfortunately lacks page numbers to refer to) lists vaporization rates for various sized containers (100 lbs being the smallest, 5 times your typical BBQ cylinder) at different temperatures on the 6th page - at 40F the 100 lb can deliver 61,500 BTU/hr, at 20 F 43,300, and at 0F 25,000  - A pound of propane is 21,600 BTU, a gallon is 91,000 BTU.