THERE ARE NO RULES.
Creating a new type for FOTA
Example: From https://bugzilla.mozilla.org/show_bug.cgi?id=1204658
The ability to specify FOTA is already in the build script: https://dxr.mozilla.org/mozilla-central/source/testing/mozharness/scripts/b2g_build.py?case=true&from=b2g_build.py#167
To change this in the build, you need to update a configuration option in the mozharness config located in testing/mozharness/configs/b2g. The existing script is: taskcluster-phone-ota.py
Probably copying that to a new script: taskcluster-phone-fota.py is the way to go.
Then a new task file needs to be added to testing/taskcluster/tasks/builds for each new build (Aries and Flame). There are a few config options in there.
And add a new variable MOZHARNESS_CONFIG in the "env:" portion of the config that points to the 'taskcluster-phone-fota.py' script.
At that point, these builds should be runnable in automation.