It will work fine with the 32 stepper. Holding torque will just be reduced as you can’t achieve the maximum amperage.
As for the couplers, you may be out of luck. There should be 3/8” shaft nema 23’s, I do believe, but if you ordered mismatched couplers and shafts it’s not going to work. That said, assuming they pick parts for the kit, I’d sure hope they provide matching stuff.