as far as I know, and I'm far from an expert, you can have as many groups as you like, so you could have:
Arches Group - all arches
Arch 1 Group - large, medium, small arch 1
Arch 2 Group - large, medium, small arch 2
Arch 3 Group - large, medium, small arch 3
Arch 4 Group - large, medium, small arch 4
Arch Large Group - large arches 1,2,3,4
Arch Medium Group - medium arches 1,2,3,4
Arch Small Group - small arches 1,2,3,4
It might be nice for us OCD people if there was some sort of grid you could overlay on the layout picture to help line up props, but the best you can get is by sight and then adjustment in the Size/Location field. As it is, your arches in the pic looks pretty lined up to me. The layout is only an approximation anyway, once your props are out they won't look exactly like your house preview anyway.
When your props are out this year take a photo and then use that for the layout pic background and you can then adjust the props on screen to match.
IMHO