Can adb sideload be used to flash/install ZIPs using 'stock' recovery/fastboot?

adb sideloading works with adb, not fastboot. Both uses different protocols and expect different data to process. Fastboot compatible archives (or ZIPs here) contain fastboot compatible images and a simple script to flash them. Recovery compatible ZIPs are usually complex as they employ scripts with various checks and preparation before flashing anything.

In addition, stock recovery requires ZIPs signed by your OEM, otherwise, they would fail during a flash. Fastboot doesn't employ such signature based check when flashing as long as bootloader is unlocked.

For understanding you can compare fastboot images with recovery flashable images from here and here.