This is the first truck I have owned with a drain plug on the tranny pan, so I was thinking: What if I changed just the fluid in the pan at every oil change (7k-8k mi) instead of changing it all every 30k. Then I would never have to go through the hassle and expense of doing all 20 qts at once. Would that work, or should I change out all of it occasionally?

Thats what we do on the plow trucks, it works well IMO to drain the pan periodicly. And since the gasket is reusable you can drop the pan and take a looksie too. Just watch the internal filter when you drop the pan, more fluid will pee out if it comes lose and the pan holds a bit of fluid even after you drain it reinstall the plug and be cautious when you drop the pan.

I think every 7-8k is a overkill. Maybe every other oil change to make milage easy to remember or every year ect......

I agree it's a little overkill. I like the spin-on changed at every oil change ( 5K miles). If you want to get more fluid out of the Ally,use air pressure in the mainline pressure port ( only plug in the bottom of bellhousing). You want the plug out of the pan,then insert air nozzle in hole and blow for a while. You can call me for better instructions.
mike