I am typing on the 12" rMB I just received a few days ago. It came with Sierra, so one of the first things I had to do was to update it to High Sierra. Thing is, from what I understand, any new machine is not going to be able to go backward to another version of the OS earlier than whatever was initially installed on it, so in this situation if for some reason I wanted to retreat from High Sierra, the furthest back I could go would be Sierra, since that is the OS which came with the machine. If I wanted to go back to El Capitan on one of my older machines, though, that would be possible, since that machine had it installed at some point in the first place.
Whatever you want on the computer in terms of configuration will have to be ordered at the time of purchase, as swapping out SSD or adding RAM later is not possible with the 12" MacBook. If you want 512 GB storage, order it at the get-go. If you want 16 GB RAM, order it at the get-go. The days when we could open up a MacBook and swap out hard drive and RAM are gone....