Can you use Harmony Elite and the Harmony Companion to control Zwave devices that are connected to the smart things hub?
It should. Anything with the home control firmware on it should work.
When you pair the ST with the Harmony App you can see the ST Devices as Harmony Devices and Yes you can control them through the App
Depending on the remote you have, you can also control the ST devices through the remote (remotes like the ultimate or Elite with a touchscreen can see device list and you will see and control your lights there)
The Smart Remote companion can not control ST devices as you can not (unfortunately) assign ST devices to the buttons. You can still include ST devices in start and end of Harmony activities, so you can Turn On/Off lights as part of the activity.
Btw.
If you do the paring on the opposite direction you can trigger Harmony Activities from ST and ST Devices, pretty cool 
I just replaced my harmony one with an elite. When I quickly hooked it up at lunch the home buttons on the remote did control the lights, now 4 hours later they don’t. In the harmony app and software ST and all devices show up and on the ST app harmony shows up in smart apps. I rebooted my midem, router, ST hub and harmony hub. Everything else in the elite appears to be working normally. Any help would be greatly appreciated. Actually just noticed the long press does turn the lights off but the short press doesn’t turn them on and unlike the other devices when I go to edit the lights it doesn’t allow me. Also when I open the harmony app and select 'outside lights ’ I can’t turn them on but I can turn them off and if I tell Alexa to adjust the brightness the adjustment shows up in the app. This combined with some lag and the need to push a button twice frequently has me thinking I might return it and stick with my harmony one for now.