It is not immediately obvious which complication families need to be implemented to support a specific watch face.
Also, which complication families can be placed in the same complication slot. For example, it appears the either CLKComplicationFamily.extraLarge or CLKComplicationFamily.graphicExtraLarge can be used in the same slot on the Extra Large watch face. Yet in my testing only CLKComplicationFamily.modularSmall can be used on the Modular face, even though it seems like CLKComplicationFamily.graphicCircular should fit as well.
Is there a full list somewhere of which families work on which faces?
I found this article that contains a table with the info I am after, but I'm still wondering if there is any official documentation about this.
I started a repo on GitHub to document this for myself. Hopefully it will help others.
https://github.com/curiousdustin/watchos-complicated-faces