I looked at iSpyConnect and MotionPie last week and they both looked really interesting. But, I wasn't sure how many cameras could be realistically be processed by a RPi. I assumed that the power requirements would make running the cameras on the same system as HG would be problematic. I figured it would be better to just buy another Pi for the cameras, but then if the camera image processing takes up a whole RPi per camera, I'd probably look another way.
As far as triggering actions, I was thinking about either using MQTT to send an alarm notice for HG to pick up or just using the API calls. Since my MQTT broker is on the HG RPi, there should be no issue with losing messages beyond what would already be seen with the API route.