Like the ideas.. just a few quick things I thought of while reading your post..
What size reservoir were you thinking of? The 18gal is about 23" wide but probably only 18.5" usable working top area. You may need a longer one? Well, if you want exact 12" spacing
I'm using the Eco633 with 22 sprayers right now and it works great! The 396 was just barely drizzling. I think 2 more sprayers will be ok. My units have 2 6' posts with 10 sites/post for a total of 20/unit. The spacing is only 6" center to center, but I can cut back to every other one if I need to...
Stink or Dirt, with their experience might have some better opinions on spacing? I would think it would depend somewhat on how long you veg for, but these hydro plants grow so fast!! I'm playing it by ear with more sites/less spacing.. will adjust as necessary.
So.. the 6" spacing between plant and sprayers.. I could be wrong here, but I think the sprayers will reach 6"... pretty close anyway. My roots are pretty massive already... they will adjust to their new environment.. I think this would be fine.
Also, I have a question--are you only using one flower unit? With three posts so you can still harvest every 3 weeks off the same reservoir/unit? You should consider putting on/off ball valves between each post so when you harvest one you can stop the flow to that post.
I haven't been this excited about growing either! Just wait... just freaking wait until you get it up and running! Its so simple.. so easy. Dirt? Never again.