So then the question now becomes,
1.) Which refrigerants are pretty much guaranteed not to condense at room temperature? R-744?
2.) Specifically, which refrigerants need very low temperatures to condense? I know that R-14 is a candidate.
3.) Should both compressors be a different size? The first one larger than the second?
4.) Could such a system function with only a single refrigerant? Or would you still need another seeding refrigerant (like R-600a) to work for the subcooler downstream from the second compressor?
5.) Could you have a combination of refrigerants all incapable of condensing at room temperature? (That seems to make sense)
6.) So which refrigerants require an even lower condensing temperature than R-744/CO2?
1,2)This is kind of andvanced stuff but anyway. You don't look for a refridgerant that does not condense in room temperature. You can do this with every rifridgerant. You just "get rid of" the gas superheating (cooling the gas, so it gets closer to condensing temperature at the spesific pressure).

3) Usually the first compressor (that sucks superheated gas from the evaporator) is close to 3 times larger than the sencond compressor. This is due to the lower spesific volume on the gass after the intercooler.

4) The difference between cascade and traditional two-stage system is that you just have one refridgerant in a two stage system. (The same refridgerant runs through the entire system). In a cascade you can have the same refridgerant, but they run in a independent loop that heat exchange (The 1st. stage is condensing against the 2. stage evaporator).

5) You chose refridgerant based on the pressure and temperature you want the system to have. Things like oil blend capability, and oil types is also a factor. The critical pressure/temp (the maximum pressure that a refridgerant can have in order to condens/evaporate) is a major factor.

6) Nitrogen has an extremely low critical point. (34bar/-147c) But this is not what you want to consentrate on.

Hope this helps!