Hmm, if I understand you correctly, the "grommets" are just the regular holes that the parts go in like any other PCB. If you're used to single-sided boards etched at home, the difference between that and these is that these are "plated through holes". The pad that you would solder to on a single-sided, home-etched board is still there, but the inside of the hole has metal plated into it to make a better connection, and so that you can solder from either side of the board, and to connect the top and bottom copper layers.
But you can solder to it just like any other kind of pcb. No special tool is required. You don't need to get the iron inside the hole, just heat the pad and component lead as you normally would with any PCB.
You'll be fine with an 18v supply. I'm using a 19.5v one and a box smaller than a standard RACO.
Your ground connection for pedals should probably be the ground tab on your power inlet jack.