Minimum Transmission Fluid Temp?

I did the modification to remove the transmission fluid lines from running through the radiator and they are currently running through the factory plate cooler. I'm planning to add an additional plate cooler along with an external spin on filter.

The questions is should there be a minimum fluid temp for normal driving. Will too low of a temp cause problems? Now that the outside temps have fallen the trans temps are only reaching 135-150 degrees with normal driving. Could that create issues with the transmission not being up to operating temps?

 

I believe the transmission will not shift into higher gears until a certain temperature is reached. I think @gearcruncher can shed some more light.