The layout of the dropdown (see below) for selecting a new action to create is a bit disorganized logically. My suggestions:
move Run Custom Action to the end of Control Switches, Push Buttons. It's yet another way to control a device, and putting it in the first device line seems reasonable.
Move Set Private Boolean up one line to replace the moved Run Custom Action. It makes sense to put all the set something actions on the same line.
Rename Run Custom Action something like Custom Device Capability. It was confusing at first to see this and then realize it wasn't about generically creating a new action, but about using special device capabilities that weren't in the standard list.
I'm with @jbaruch , it's not just switches or buttons where you may want to run a custom action, could be any device.
Perhaps naming it Run Custom Command would fit more closely to how they are presented on the Device Detail page. I don't expect the term capability is one many users would be familiar with.
Looking at the list, it feels like moving set variable down the the private Boolean group may be a better grouping. Although I just re-read your suggestion on this... Either or....
