"New" calipers from them have good rep in most posted here.
Good Rear calipers should auto adjust, AKA Self Adjust, w/o problems. Methods to Manually adjust is often trouble now or soon after and I said this in my cave... See my Cave, Rear Brakes So if that or any other methods works, call the vendor to return under warranty.
Rear calipers, more accurate the rear Pistons, can hold air and may need help to shake it loose from inside of the pistons. Used plastic/wood to gently hit the calipers.
Note: Sliders need proper lube. See my Cave, Brake Service 88 Slider's dust shields are the "return spring" for the caliper outboard pad and can fight you w/o problem lube sim to other slider "o-ring" for older Fiero. Lack of lube can seize the caliper later too as water etc corrodes the aluminum.
Both Rear calipers off/open @ same time can drain tube between the two and can take longer to purge air to left side.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave