The traditional approach with an auxiliary cooler is radiator first then cooler - mostly to get best cooling in high temp/high load situations. I think 15x7 is a little small to handle cooling duties all by itself thus the recommendation to run them in series to prevent cooking the transmission.

Newer cars like gen II and gen III Durangos have a giant cooler in front of the radiator and no radiator bottom tank cooler. Maybe the transmission has an internal thermostat for those -10F days. In any case it works fine in cold weather.

I believe Dodge used an external thermostat for trans oil on on certain year PU's (90's?). From what I have heard, it had a habit of failing closed which cooked the transmission.