Mel,
The biggest difference between active dry yeast (ADY) and instant dry yeast (IDY) is that the IDY doesn't have to be rehydrated ("proofed") in water. It can just be tossed into the flour. Also, less of the IDY is needed to achieve the same leavening power as the ADY and the IDY can tolerate higher water temperatures than the ADY, which should promote faster rising if that is the objective. The "rapid rise" yeast is basically an IDY that is marketed more to consumers than to professionals, but can also be used although it is a different strain than the IDY that is sold to professionals. If you plan to do a lot of pizza making, using either ADY or IDY, my best advice is to buy a 1-lb. bag. It is far cheaper over time than buying the small packets at the supermarket. I understand that places like Costco's and Sam's sell the bags; they can also be ordered by mail order from several places.
FYI, one packet of ADY = 1/4 oz. = 2 1/4 t. = 1 (0.6-oz.) fresh compressed yeast cake = 1 1/2 t. IDY. So, to convert from ADY to IDY, divide the amount of ADY by 1.5.
Peter
