Don't know about the others, but, of you're flying business with Thai then you don't need a fast track pass - you just have to show them your business class boarding pass. Have done it more often than I can remember over the last four years...
When I travelled business on Thai they did not have a voucher but did have a separate area, just showed the boarding pass stub. Was actually less crowded than the fast track. Don't know about the others. It was in the normal immigration area on the right hand side.
I would pay 700 baht to avoid the queues, since the oil price slump I fly economy most of the time. I miss fast immigration service I use to get when flying business.
Edit. ausmagoo beat me to it.
If you use Thai.... on the departure you just go to the business/first class desks designated for Thai Aiways travel and you do not go near the other Immigration Lines... it has it's own one.
On arrival into Thailand.... if you are travelling business/first class with Thai...… you use the priority lane by showing the stub of your boarding pass.