Rather than using a Z wave contact sensor, you might want to look at ST_Anything. It is kind of perfect for what you are looking at here with the multiple inputs. It’s also a heck of a lot cheaper and more customizable. That would allow you to report back the status of your alarm system to ST.
How would you arm the system from SmartThings? You can’t use a contact sensor to do anything…it’s a sensor. You would need a relay of sometype. Again, I think ST_Anything would work well here using the “switch” capability. A contact sensor would work if the Alarm is changing the state to report back to ST but it’s not going to change anything to arm your system.
With ST_Anyting, you could have all 8 relays wired up to one device that costs around $9 instead of having to but 4 z wave contact sensors and 4 z-wave relays.