If you have an 01 or newer Ford truck, it should have a bypass valve to route the fluid back to the trans until the fluid warms up. Install the aux cooler down stream of the factory installed cooler, but prior to the bypass output, and it will work the same. In other words keep it in the same loop as the factory cooler.
Ideal temp is 165F+ not towing, max 210F towing. Over 210F, the fluid life starts to get shorter much quicker. Over 220F for any length of time, time to look at a change. If I hit 220F, I pull over ASAP if traffic allows. Others on here will give you other temps, but the load, speed, wind and hills will determine part of the temp. If you are seeing 190+ degrees empty, you need more cooling. IMHO.
As you stated, heat is a killer on trans fluid. Use a good fluid and change at 25K. Mobil 1, Amsoil, Schaeffer's, Pennzoil, Chevron and many others.
I haven't seen many complaints of failing transmissions for running to cool. If I had to err, I would do it on the cool side on my trans.
