I wouldn't worry about the trans fluid getting too thick. All auto trans missions have a "cooler" in the radiator. It doesn't only cool the fluid down when it's too hot but warms it up if it's to cold so it can maintain normal operating temp. Now an aux trans cooler is supposed to be installed before the cooler in the radiator. So fluid comes out of trans, gets cooled in the aux cooler, then it is brought to operating temp in the cooler that's in the radiator, and then it goes back to the trans.
2011 Chevy Silverado LT, Crew Cab 4x4, 5.3l
2003 Custom 189fbd FunFinder.