I'll just echo running a dedicated / home-run for the alarm. If you just can't do it you'll have to go the other route (back-feed / 2nd pair), do test the circuit you'll be using for faults, daisy-chains, etc.
Also as stated, the customer should test said alarm before the install. You'd be surprised at how many are not working correctly prior to an install.
Mock ups are very good training aids and if you construct some make sure you create a few challenges for your techs
