Ya, most people dont have r-12 sitting around in their garage, and they dont come in 12 oz cans, they would be more like a gallon bottle.
If you follow the instructions on the refill or retrofit kit you will be fine. Since you are only low and not empty, just leave the can right side up as stated in instructions. Ive done this many times and also had the pressure verified later and all was fine. Leaving the can right side up only allows just enough refrigerant to pass into the system. Putting the can upside down like when you are doing a complete refill will put the entire can in the system.
To get to the codes you hold off an warmer, and when PCM? is displayed use your fan hi an lo buttons to scroll through, you may have codes in different categories to clear, such as IPC, RSS, and so on, just clearing one category only does that category.
The A/C code will be a APC code such as AO46, AO47, or AO48