Thermal Expansion Tank(s)?

Hi everyone. This is my first time here and I really hope you can help me out. I have four fifty gallon HWHs all feeding off of a single cold water line 1" diameter copper reduced to 3/4' PEX to the inputs and outputs. A while ago, a water softener was added to the system and I've just recently noticed that one and perhaps two of the hot water tank T&P valves have been leaking (much green oxide). The installer of the softener told me there is a check valve in it to prevent back flow. I'm guessing that 200 gallons of water being heated is expanding enough to trip the T&P valve(s).
I have visited Amtrol's and Watts' web sites and have determined that there is an expansion of 2.19 to 2.4 gallons of water in my system as the water heats up. My question is this, can I use just one expansion tank on the line? Notes: Three of the HWHs are pretty much right next to each other and the final one is about 25 feet away from the other three. I'm thinking about using one Watts PLT-12 or one Amtrol Therm-X-Trol ST-12. House H2O pressure: 55 psi; Max pressure: 150 psi; H2O temperature: 130 F; Total capacity of HWHs: 200 Gallons; Water source: Drilled Well with Water Softener AFTER main expansion tank.

Whenever you have a closed system, you need an expansion tank. Often, the bladder tank on a well can perform that task, but if you have another check-valve, like with your softener, you need one 'in that loop'. Doesn't really matter where it is as long as it is on the COLD line, after the checkvalve. Code wants it after the shutoff to the tank, so it can't be isolated, and can absorb that expansion.

Thank you!