Not sure if 'gang box plugs' are the same as the "Hole Plugs" which I use.
Yes, same thing. Couldn't think of what else to call them off the top of my head. My typical use for them is plugging holes in gang boxes and other metal electrical entry ways.
Regarding the ball valves, I'd personally probably split the difference and put two towards the front and cap the back ones. That way the majority of what is needed will have fine control without mucking around removing caps. And if it comes to it, then I can stop being lazy and remove a cap from the back. But I'm not a UDS expert; I'm more just discussing in terms of how I would approach it.