I saw that recently Stack Lighting has opened up their API. I am curios if anyone is working on something for these to talk together or not. I know Stack Lighting uses ZigBee. And the API list looks like it might be decent. I do not have the strongest skill set for this but I can maybe try and do something. I figured I would see if anyone else knew about this first.
If it’s an open API, it shouldn’t matter what protocol that uses on its own, although you will need its own controller to make it work.
I’m sure people here can help you with setting up a cloud to cloud integration. 
If you want something simpler, they do have an IFTTT channel.
They do not work with the hue bridge (they recommend IFTTT integration instead), so I don’t think it’s going to work directly with SmartThings.
The zigbee standard allows for different profiles, and they can’t all talk to each other. So just being zigbee isn’t enough to be compatible with SmartThings.
But again, the protocol won’t matter if you use the cloud to cloud integration.
All valid points. The IFTTT is delayed and offers limited options which isn’t ideal. They give you an option to turn on; when occupancy is detected. But no options to turn anything off.
But like you said maybe someone here can help. I will start to tinker with it some. It rely on the REST API; and send commands of HTTP. Looking up the Smartthings settings It might work. But I am no expert on this.