There are a couple variables to consider here. One, downgrading a Zebra Android device to a lower Android and security patch level will result in the device being factory reset. It is therefore a strong possibility that you will orphan the devices which would force you to have to manually re-enroll them to get back into SOTI. For this reason I personally would recommend downgrading them from 11 to 8 BEFORE you enroll them into SOTI using a mechanism like StageNow.  

The second variable to consider is that there may be hardware constraints that actually prevent you from being able to downgrade. Meaning, that even though there are Android 8 builds available for the TC57 if you have a brand new device there may be new hardware components that do not support older versions of Android OS, depending on the BSP. I do not know if this is the exact case for the TC57, but many of the Zebra Android devices will prevent downgrades if they feature newer hardware components. You should verify that you can manually downgrade one of the devices using a separate method like StageNow before investigating whether or not this is a SOTI issue.

Third, and finally, if you are still wanting to downgrade the devices via SOTI, despite the factory reset potential, AND the devices are actually downgrade eligible, then you will very likely need to use the MX based Downgrade command rather than using the SOTI based install_system_update script to actually process the downgrade.

mxconfig /sdcard/<MXFILECONTAININGADOWNGRADEINSTRUCTION>.xml
 

There is alternatively an intent based mechanism that could work but I would suggest the MX XML approach.