If there's R12 in there, you should not have added any R134a. Sorry to say, but if that is the case, you will find it nearly impossible to even take it to some schmo for removal.

The two refrigerants need oils that do not even resemble each other, so it is important to use the correct refrigerant. If you charge with 134a and are using mineral oil (intended for R12), you will ruin your compressor due to oil starvation. If you charge with R12 and have PAG oil, you will ruin your compressor due to acid formation.

If it is a 134a system, it is required to have a high-side cutout switch to prevent the compressor from venting. R12 systems do not have this requirement, and the old R12 systems didn't have the switch.

As for the "correct" pressures, that depends on a whole host of factors. The charts are in the service manuals.