Fix-a-Flat should only be used if there is no other alternative, IMHO.
I say that because it leaves a huge amount of messy goo inside the tire that will inevitably cause the tire to be VERY much unbalanced, thus rendering the car undriveable at high speed.
Re-balancing that tire properly would require removing the tire from the wheel in order to remove all of that goo before balancing can actually be done. Many shops will impose an extra charge–over and above the balancing fee–for removing all of that revolting mess from inside your tire.
So–if I had cannibals rapidly approaching my disabled car, I guess that I would use F-a-F, but in other, more common flat-tire scenarios, I would simply opt to install my temporary spare and drive to a tire shop for proper repair of the flat tire. Proper repair involves removing the tire from the wheel, removing the nail or other object, and installing a combination plug/patch from the inside of the tire.
As to the other question, texases is correct, and you really need to read your Owner’s Manual in order to find out this information, as well as lots of other important information regarding the safe and economical operation of your car.
However, if you can locate a tire shop that will “shave” the tread of a new tire so that it matches the circumference of your old tires, you can simply buy one tire. I suggest that you locate such a shop now, before you are in the situation of having to locate one on the spur of the moment.