can I monitor transmission fluid temp with scan gauge

All would depend upon if there's a sensor for it already, I'm going to assume there is but one may never truly know. If it DOES have the sensor the Scan Gauge and Ultra Gauge will both recognize it and allow you to monitor it.

 

The transmission fluid doesn't get very hot, but is cooled anyway with the inverter coolant. Monitor the inverter coolant pid.

GT-I9300 ?

 

While the original Prius hybrid system had the inverter coolant loop going through the transmission, I believe the new (post 2010) system leaves it out, and the inverter coolant loop cools only the inverter components, and the new transaxle heat dissipation mechanism is conduction through the fluid to the case and convection/air flow around the case..

This explains why on my 2001, my MG temperatures never got much above 60-65C except on really hot days, and there was always a few degrees difference between MG1 and MG2, but on the 2013, I'll routinely see 85-90C temperatures (ie, same temp range as the engine block) on both MG1 and MG2 with very little difference between them, even in the winter, as soon as the car has been operating a while..

 

If I saw the diagrams correctly you are pretty close. The Trans fluid is cooled like an AT car through a small core in the bottom of the ICE Radiator and the Inverters coolant is it's own separate system.

 

The Prius C’s transaxle is 16% lighter than the liftback’s by eliminating the air-fluid-cooling radiator system. The rotation of the oil pump cools the automatic transmission fluid in the Prius C, as noted by c4 above.
Eye-balling the radiator shows 2 connectors for the ICE and 2 for the inverters at the base.
As for the OP's question, like the ICE oil temperature, the trans fluid temp may not have a sensor, one that's available anyway.