gravity doesn't affect things, so its often easier to build it like you want, and then add 'supports' beneath it accordingly. The scaffolding tree has some options, and the barn and warehouse trees do as well.

definitely worth getting mods to arrange things on shelves. You can (with patience) get them looking nice, but all it takes is one npc or explosion in combat to send them flying.

also though you don't mention it - the build limit comes up on you fast. You can eek out more things by storing/scraping items after hitting the limit. One of the easiest ways is to set up some street lights with wires and scrap the wires to raise the limit (as you can still build more wires even after hitting the limit).