I always make a template of my tank footprint on a piece of ply, or cardboard may work, and I play w/ the rock to see what fits best in that plotted space.
I personally like to drill my rock and use rods to make it stable, but epoxy could work too.
I make mine in movable pieces or islands.
I also cover the glass both inside and outside front panel and sides w/ carboard so I don't scratch it placing rocks in.
I like to lay down about a half inch of sand to seat that rock and deal w/ the voids or deviations, place rock to where it looks good, then add rest of sand to lock in place.
Yes tap is only a little dirty water I guess, but I don't want to add even a little of what I will be trying to remove later, so I use clean RO/DI to rinse that sand.
I pump water in w/ a pump and hose, and I place tupperware or something so I don't dust storm the sand.