I have the Skene P3 lights on both my FJR and my Hondapotamus. The P3 s use the same controller, which should speak to the size of the unit.
I can't comment on the use of the controller with existing or add-on lighting, but my experience with the controller & P3 lights should be similar:
I installed the unit under the rear seat. The unit is about one third the size of my iPhone.
The instructions are very clear and easy to follow, and all needed parts and connectors are supplied. Installation was simple!
I've had the unit in my Wingabago for 8+ years with no issues - works flawlessly. The unit in my FJR was installed in early July of this year.
I wouldn't own a bike without it. Period.