Hi, i've got my radiator broken and the transmission oil mixed with cooling liquid and it caused transmission broke! Now i must replace transmission, and obviously i must repair the radiator (however the engine temp is always ok, it's only the mixing of fluids). the question is: change radiator or bypass it and buy a bigger transmission cooler with a fan? because now i'm scared that a new radiator can broke again,and it will cost me another tranny! but i don't know if bypassing the radiator the tranny liquid will be too cold.. can someone help me please? thanks

Ford f150 5.4 v8 2004 fx4

 

I would just replace the radiator. It is fairly rare for them to cross contaminate like yours did. You will need to replace the radiator anyway, because it will start pushing antifreeze into the trans cooler area and cause leaks and other issues.
I have never had issues on older vehicle with just an external cooler with the fluid being too cool, you just want to make sure if you go the separate route, that it is a big enough cooler for the maximum combined vehicle weight.