Each unit places magnets on both sides of the pipe itself.
Basically that depends on your water source and water quality. For municipal water, no, you don't normally need a whole house filter. But a filter for the refrigerator and a Pur Water filter water may help you ditch the bottled water habit. Private well water (or any water) with high iron content will benefit from the addition of iron filters also. .
By stacking additional units on the incoming water line you extend the contact time your water has with the magnetic field. The number of units recommended is based on the Total Hardness of your water. If the Total Hardness of your water is 10.5 Grains (180 ppm) or less, one unit on the cold water source may be all that you need. If it ranges between 10.5 and 15 Grains (180 to 257 ppm) we recommend two units on the cold water supply. You could use three units on the cold water supply if your Total Hardness exceeds 15 Grains (257ppm). Three units on the incoming water supply is the maximum number we recommend. After that I feel that you have accomplished all that can be accomplished with magnet treatment.
It's not where do they go, it's where they don't go. The minerals stay in the water. When you flush the commode they go down the drain. When you drink the water you also get the minerals that your body needs. Your body probably does not need to have these minerals replaced with sodium from a salt-based water softener. Few Doctors will tell you to add more salt to your diet.
Since the steel frame is a magnetic shield itself, I do not think that would even be a concern. Six inches would be more than enough spacing even if there were some problems with the magnetic field.