Installing transmission cooler today, bypassing radiator.

Just replaced my serpentine belt and lubricated the heck out of the belt tensioner, no more chirping! woo! Also... I found a leak in the smaller hose that goes into the throttle body, I'm unsure of what it is, but I replaced the hose, I don't know what sort of problems that could have caused.

Well, I've decided I'm going to bypass the transmission cooler built into the radiator today and route those hoses to a 9"x9" transmission cooler and mount it behind the grill.

Reason: I've read too many horror stories of the cooler built into the radiator leaking radiator fluid (engine coolant/antifreeze) into the transmission cooler lines, thus leading to transmission failure and a big fight with nissan to get it repaired.

Concerns: Let's say I add about 3 ft of hose total, how much transmission fluid will I have to add to the transmission after the new cooler fills with fluid? I just want to be prepared so I can go get transmission fluid before I begin. It was last serviced by nissan, so whatever the heck nissan uses for transmission fluid is what is in it.