...but could a soldier also carrier multiple sights like an ACOG (my favourite) for ranged combat and a holographic sight for close quarters Combat? And switch them via the pinnacity rails?
You can slap on as many sights as you want. You can slap on a high powered scope on the top, a red dot on the side, and 2 ACOGs on the bottom if you wanted to. It won't be practical at all, but it is possible.
And most mounts/sights will not hold zero if you pull them off. So if you pull off an ACOG to put on an Eotech, the EOtech will not be zeroed. In other words, it will be useless. Likewise, you can't just put your ACOG back on as it would have lost zero to. Not all mounts are quick detatchable either. Some of them require a screw driver. Even for those sights that you can quick swap, you generally wouldn't do it during battle.
Though some people do use multiple sights. A lot of people use ACOGs as their primary, and put a canted mount on the right or left of the rifle and put an Aimpoint Micro there. When they get in close ranges, they tilt their rifle sideways slightly to use the close range sights. That said, it is not that common.