Lets examine exactly what Turret, Vehicle Mount, and Fixed mean compared to the ACS Turret.
First I agree that the intent in the Beta was that you could drop an ACS Turret into a VehicleMaker Weapons Mount, if I recall correctly it explicitly said that. I'd also agree that you should be able to take GunMaker weapons and mount them in ACS mounts.
The problem is that somewhere along the road to publication the importance and separateness of Turret, Vehicle Mount and Fixed seem to have been set aside in VehicleMaker.
VehicleMaker makes reference only to "Weapon Mount" as an installable component on p.300. Yes it does refer to vehicle mounts, turrets and fixed mounts, but only as pre-installed components on Military Vehicles ref. p.298.
They way I've been treating this is to use the definitions as a guide to what I'm actually designing. For example: I have a 1ton volume weapon mount installed on a Vehicle. I take this to mean that I can install 200kg to 100,000kg of weapons systems in that mount. If I install one big cannon of 500kg I understand that my vehicle has a turret in that weapons mount. If i install two or four very heavy machine guns massing 400kg say, then I understand that I have a twin or quad turret on my vehicle. In my head half the ton volume is inside and half outside like a small vehicle turret in the real world. If I install an oversized autocannon of 600kg+ then thats a vehicle mount and in my head suggests a modern MBT turret comprising a 1ton block on top of the hull. A really big weapons system like a pair of os autocannon or an ICBM modeled on an ACS missile needs a fixed mount which to me suggests the kind of turret you get on a modern self propelled gun or a missile erector/launcher like that found on a mobile ICBM launcher vehicle.
In other words turret in GunMaker does not directly equal a turret in ACS.
There is a corollary to this. An ACS Turret may have a volume of between 0.333tons and 4tons. You couldn't drop that directly into a VehicleMaker Vehicle Mount which is always 1ton.
Stated another way; Turret, Vehicle Mount and Fixed from GunMaker are all expected to have a volume of 1ton. The min/max mass parameters on p.253 are there to help you decide how many weapons fit in the 1ton volume and to interpret what kind of mounting is on your vehicle. Remember there are no rules on the subject of different arcs of fire for these different mounts, so its more of a design parameter thing.