I was wondering if I retrofit 4th gen caliper brackets/brake rotors on a 5.5th gen, would I be able to downsize to 15inch to get 23.5/8.0/15 for drag racing?

Pretty sure that depending on what the offset of the supposed rims are WITH the correct spacer you should be able to run them with the stock brakes.
What rims are you looking at?

The lightest possible I can fit, I just want to have 15 inches because of the slick size choice I'm looking to run (23.5/8.0-15 E.T drag radials)

If I can figure a width and offset layout that can be installed on stock brakes, with spacers to clear the outer edge of the caliper, i'd be pleased.

15x7 0 offset 5x114.3mm would probably fit. Black steelies would do the job...

Spacers will not help fitting 15's over 5.5gen brakes.

Yes 15s will fit if you install 4th gen calipers and rotors. But then you have the issue of the different bolt size for the hub to torque member bolts. You could either drill and tap the torque member for the larger bolts, or find/have made a concentric spacer that will center the smaller 4th gen bolt in the 5.5 gen hub.

Or you could go to town with an angle grinder on the 5.5gen caliper. We chose this option on my buddies 5.5 gen when we were installing a set of millenias on it.

I found a set of ford ranger rims on 15x6.5 7offset for 51$ each brand new. I might try to fit them on the car just to see the space I have and work to do on the caliper shaving

I confirm that Ford Ranger rims in 15x6 5x114.3mm +7 offset bolts on directly without ANY modifications on oem brake size on 5.5th gens. My part number is X40709 from Macpek, Canada.

Nice pioneering. Next 23.5/8.0/15 gummies from my friend Mickey thompson.

How much do those wheels weigh?

steel 15inch rim...not weighted but not that light. say 15-16lbs. anyway im gonna try these on with slicks in 23.5/8.0-15