Dialing in coilovers can be a pain, I definately learned that. But there is no doubt that they are wonderful. With a 12" of travel, you will more than likely be fine not running triple rate. You will need the sprung weight, angle of the shock to the lower link, your desired droop, length of your lower links, and length to your lower shock mounting point. You should probably read up on coilover tech, I would recommend Billavista's Tech on Pirate. As far as bumps, all they are is bumpstops. So unless you are bottoming out, there is no use for them. You could spend quite a bit to get the correct springs at around $50 each, but most companies sell springs also have a service where you can return them if they are not what you need and they are not torn up. Make sure of this when you order, it well help a lot. There is a ton of information out there on 4 link suspensions fortunately.
Where does FOA say that about the lengths of the springs? That recommendation doesnt seem to make much sense to me since that ould mean the preload could not be adjusted up. Most setups I have been looking at use a spring set up that put the preload at the middle setting to allow for adjustment of both the preload up and down as well as the transition between the main/ tender.