In many states, individual metering of water/sewer and the like requires you to be under the umbrella of that state's public utility commission.
The local water authority may be able to work out an arrangement to meter an bill your tenants directly if the lease doesn't prohibit this. However...
AWWA C515 covers larger gate valves, but I agree that the largest AWWA conforming RW GV is 48". I believe you can still get Double disc as large as 72"
http://www.charlesfinchvalve.com/ard.htm
Also to consider that a gate valve can be rebuilt without removing it from the line. An MJ butterfly valve cannot. My experience with BFVs is that they usually fail in the open position, and therefore are useless when you really need them. The only way to make the valve servicable is to use a...
generally it is specified at no direct pay.
NOTE: make sure you are not using a butterfly valve in this line. only a gate valve or other similar full port valve will work
I don't see where you would need to throttle a 6" valve to drain a 72" main. It is going to take a long time to bleed pressure off of a main that large with a valve that small. The last application of a drain gate valve that I was involved in we used a 12" valve to drain a 48" main. With...