Mixing R22 with R407C or any other refrigerant.
According to the 609 EPA rule, mixing refrigerants is illegal and anybody caught doing so will be heavily fined.
Although you may this restriction is primarily political we would like to give you a basic education to explain the two most devastating effects of such action:
To the environment.
R22 is a HCFC chemical gas that create a chain reaction in the stratosphere and thin the ozone layer that protect us from the UV light of the sun. When you mix this chemical with any other refrigerant (most likely an HFC) you create an imbalance on the final substance that can result on even more dangerous effect on our planet.
Once HCFC and HFC are mixed into your system will be impossible to re-separate (unless processed through a specific reclaim facility) the individual components and most likely you will end up with the need of incinerate the bland (extremely expensive procedure!).
To your AC System.
If your system is running on R22 most likely is using mineral oil to lubricate the compressor. If you mix R407C with R22 inside your system the molecule of the HFC (R407C) gas will not link with the oil and in just few months of operation you will have a "sludge" like lubricant formation inside the circuit. No need to say this will result in catastrophic failure of the compressor and most likely you'll need to replace the whole unit.
There are "drop-in" R22 replacement in the market like R421a, R422B and MO99 (also MO29 for low-temp applications) but NONE of those can be mixed with R22.
The bottom line is: If your system lost more that 15-20% of the total R22 charge you may want to consider to evacuate (with a recovery unit) the residual gas and recharge with a replacement. Anything below this amount you may want to stick with R22.
Kim, S. G., M. S. Kim, and S. T. Ro. "Experimental investigation of the performance of R22, R407C and R410A in several capillary tubes for air-conditioners." International Journal of Refrigeration 25.5 (2002): 521-531. [1